What the Filioque Actually Says
The dispute is about the eternal procession of the Holy Spirit within the Trinity — not about the Spirit’s activity in the world or in salvation, but about the inner life of God Himself. Does the Holy Spirit proceed from the Father alone, as the Orthodox hold? Or does He proceed from both the Father and the Son as from a single principle, as the Catholic formulation states?
The Nicene Creed, as defined at the First Council of Constantinople in 381 AD, states that the Holy Spirit “proceeds from the Father.” This language is drawn directly from John 15:26, where Jesus says: “When the Advocate comes, whom I will send to you from the Father — the Spirit of truth who proceeds from the Father — he will testify about me.” The Greek Fathers, working in the tradition of Cappadocian theology, took “proceeds from the Father” to mean that the Father is the sole principle, source, and cause (aitia) within the Trinity from whom the other two Persons have their being.
The Latin West, developing along different theological lines, added Filioque — “and from the Son” — to emphasize the consubstantiality of Father and Son and to counter lingering Arian tendencies that subordinated the Son to the Father. If the Spirit proceeds only from the Father, some feared this implied the Son’s inferiority. If the Spirit proceeds from both, the Son’s full divine equality is secured.
How It Entered the Creed
The Filioque was not inserted into the universal Creed by any ecumenical council. It first appeared at the regional Third Council of Toledo (589) in Visigothic Spain, where it functioned as a doctrinal hedge against residual Arianism among the recently converted Visigothic ruling class. It spread through the Frankish churches in the seventh and eighth centuries, was championed at the court of Charlemagne, and eventually reached Rome in the early eleventh century, where it became standard in the Roman Rite.
This procedural reality matters significantly. The Council of Ephesus in 431 AD had explicitly forbidden additions to the Nicene Creed. Inserting the Filioque — whatever its theological merits — was a unilateral act by the Western church without the consent of the East.
When Frankish monks in Jerusalem were challenged by Eastern monks over the Filioque around 809 AD, Pope Leo III was consulted. He affirmed the theological content of the Filioque as correct — but refused to authorize its insertion into the Creed. He had the original Creed, without the Filioque, inscribed on two heavy silver tablets in Greek and Latin and placed at the tomb of St. Peter.
This remarkable episode demonstrates that even Rome once recognized the procedural violation involved, even while accepting the doctrinal claim.
The Orthodox objection has therefore always operated on two levels: procedural and doctrinal. The procedural objection — that no single patriarchate can unilaterally alter a text defined by an ecumenical council — is widely conceded even by Catholic scholars. The doctrinal objection — that the Filioque itself teaches heresy about the Trinity — is considerably more contested, and it is here that the most significant ecumenical progress has been made.
The Photian Controversy and the Orthodox Case
The Filioque became a central flashpoint during the Photian Schism of the ninth century. Patriarch Photius of Constantinople, in his Mystagogy of the Holy Spirit (867), argued that the Filioque was not merely a procedural violation but a genuine theological error that compromised the monarchy of the Father — the doctrine that the Father alone is the unoriginate principle from whom the Son is begotten and from whom the Spirit proceeds. If the Spirit proceeds from the Son as well, Photius argued, then either there are two principles within the Trinity (destroying monotheism), or the Father’s unique role as source is compromised.
This became the definitive Orthodox formulation. The Spirit proceeds from the Father alone (ekporeuetai ek monou tou Patros). The Son has no role in the Spirit’s eternal procession. Any language suggesting otherwise is heresy.
The technical Greek term is ekporeusis — eternal procession from the Father as ultimate source. This is distinct from proienai or proodos — the Spirit’s proceeding forth or shining forth in the economy of salvation, in which the Son does play a role. Orthodox theology holds that Filioque theology collapses this distinction, attributing to the Son a role in the Spirit’s inner-Trinitarian being that belongs to the Father alone.
Vladimir Lossky and the Modern Orthodox Case
The strongest twentieth-century formulation of the Orthodox dogmatic objection comes from Vladimir Lossky (1903–1958), the Russian émigré theologian whose Mystical Theology of the Eastern Church (1944) shaped Orthodox theology for two generations. Writing from Paris in the aftermath of revolution, Lossky was no anti-Western polemicist. He engaged Catholic theology with respect, rigor, and serious scholarly knowledge of Western sources. His verdict on the Filioque is therefore all the more weighty:
“The dogma of the procession of the Holy Spirit from the Father alone is the sole dogmatic foundation of the separation of East and West. All else — including the question of the papacy — is theologically secondary.”
For Lossky, the Filioque is not a procedural irritant or a regrettable misunderstanding. It is a theology of the Trinity that, by introducing the Son into the Spirit’s eternal procession, fundamentally shifts the balance between the divine Persons. The result, in his analysis, is that Western Trinitarian thought becomes essentialist — emphasizing the divine essence shared by all three Persons — while Eastern thought remains personalist, preserving the irreducibility of the three hypostases within the unity of the divine essence. This difference, Lossky argued, ramifies through the entire theological system: it shapes anthropology, ecclesiology, and the doctrine of grace.
The Lossky thesis is contested even within Orthodoxy. His contemporary Sergius Bulgakov took a more reconciliatory approach in The Comforter. Modern scholars like A. Edward Siecienski, in his magisterial Oxford study, have argued that Lossky overstates the systematic divergence and reads later polemics back into the patristic period. But the Lossky case is the strongest version of the Orthodox dogmatic objection — and any Catholic response that engages a softer formulation has not engaged the real challenge.
Augustine and the Latin Trinitarian Tradition
Lossky’s deepest charge is not procedural. It is genealogical. The Filioque, in his analysis, is the dogmatic surface of a Western Trinitarian theology rooted in St. Augustine — a theology that begins from the divine essence and only secondarily considers the three Persons. Eastern theology, on his reading, begins from the Father as personal source and only then considers the shared essence. The result is two fundamentally different mental pictures of the Trinity, and the Filioque is the visible doctrinal symptom of the deeper essentialist drift.
This is a serious charge that deserves direct engagement. Augustine is not just one Latin Father among many; he is the architect of Western Trinitarian thought, and his De Trinitate is the single most influential treatment of the doctrine in the entire Latin tradition.
“The Holy Spirit, therefore, proceeds principally (principaliter) from the Father, and, by the Father’s timeless gift to the Son, proceeds in common from both.”
The Latin word that matters here is principaliter — “principally,” or “as from the principle.” Augustine is not collapsing the Father and the Son into a single undifferentiated source. He is asserting that the Father is the original principle of the Spirit’s procession, and that the Son’s participation in that procession is itself a gift from the Father. The Spirit proceeds from the Father as source, and through the Son by the Father’s gift. This is not Greek terminology, but it is structurally the same theology that Maximus will defend in his Letter to Marinus two centuries later.
The Council of Florence, in 1439, made this explicit. Its decree on the procession defined that the Spirit proceeds from Father and Son tanquam ab uno principio — “as from one principle” — and unica spiratione — “by a single spiration.” The Father is not displaced as the fontal source; the Father and Son together constitute one source from which the Spirit proceeds, with the Son’s participation derived from the Father’s eternal gift. This is the dogmatic formulation that the 2003 NAOCTC consultation found to be substantially compatible with the Eastern dia tou Huiou tradition.
The deeper question Lossky raises — whether Augustinian Trinitarian theology is essentialist in a way that ramifies through Western anthropology, ecclesiology, and grace — is real and ongoing in scholarly conversation. But that question is distinct from the dogmatic claim about the Spirit’s procession. Augustine’s principaliter language preserves the Father’s monarchy. Florence canonized that preservation. The two-causes charge that Photius made in the ninth century, and that Lossky systematized in the twentieth, does not survive a careful reading of either Augustine’s text or the magisterial formulations that follow from it.
Maximus the Confessor: The Patristic Bridge
The most significant patristic evidence against a clean East-West binary comes from St. Maximus the Confessor (c. 580–662), one of the most revered theologians in the entire Orthodox tradition. When challenged by Eastern critics of the Latin Filioque in the seventh century, Maximus explicitly defended the Western position — on Eastern terms.
Maximus argued that the Latins do not mean the Son is a second independent cause of the Spirit’s being alongside the Father. They mean the Spirit proceeds “through the Son” (dia tou Huiou) from the Father as sole principle. The Son is not a second source but a mediate principle — the Spirit proceeds from the Father through the Son, with the Father remaining the one ultimate cause. Maximus treated this as compatible with Eastern theology and as a defense of Western orthodoxy against unfair charges.
“The Latins have shown that they have not made the Son the cause of the Spirit — they know, in fact, that the Father is the one cause of the Son and the Spirit, the one by begetting and the other by procession — but that they have manifested the procession through Him, and have thus shown the unity and identity of the essence.”
A revered Eastern Father — venerated as a saint in the Orthodox Church, condemned to mutilation and exile by a heretical emperor for defending Chalcedonian Christology — explicitly affirms that the Latin Filioque, properly understood, does not teach two causes in the Trinity. He wrote this in defense of the Latins, in Greek, two centuries before Photius declared the doctrine heretical. His authority within Orthodoxy is of the highest order, and his judgment cannot easily be dismissed as Western sympathy.
Other Eastern Witnesses
Maximus is the most striking pre-schism Eastern witness for compatibility with Latin theology, but he is not alone. Several other Greek Fathers used formulations that Catholic theology regards as substantially Filioquist or Filioque-compatible.
Cyril of Alexandria (c. 376–444) repeatedly described the Holy Spirit as “the Spirit of the Son” and as “proceeding from the Father through the Son.” In his Thesaurus on the Holy and Consubstantial Trinity, he wrote that the Spirit “exists from the Father and the Son” (huparchei ek tou Patros kai tou Huiou) — a Greek formulation that comes remarkably close to the Latin Filioque.
Epiphanius of Salamis (c. 310–403) in his Ancoratus argued that the Holy Spirit is “from the Father and from the Son” and “from both” (ek amphoteron) — language that, taken at face value, anticipates the later Western dogmatic formulation.
Didymus the Blind (c. 313–398), the great Alexandrian theologian whom Athanasius admired, wrote that the Spirit “proceeds from the Father and the Son together” in his De Spiritu Sancto, preserved in the Latin translation of Jerome.
John of Damascus (c. 675–749), who composed the most comprehensive systematic theology of the Greek patristic tradition, taught that the Spirit proceeds from the Father through the Son (di’ Huiou) — the Greek formula that the 2003 consultation, the Catechism, and Maximus himself all treat as substantially equivalent to the Filioque.
A. Edward Siecienski’s definitive scholarly study, The Filioque: History of a Doctrinal Controversy (Oxford, 2010), summarizes the patristic evidence as far less polarized than either traditional Catholic or traditional Orthodox accounts have claimed. He concludes that the patristic record contains “no citable basis for the claim historically made by both sides” that the Greek Fathers explicitly either supported or denied the later dogmatic theologies. The Fathers were doing something earlier and more pastoral than either Photius or Aquinas would later attempt.
The 2003 Consultation: “No Longer Church-Dividing”
The most decisive modern development came in 2003, when the North American Orthodox-Catholic Theological Consultation issued a landmark agreed statement: The Filioque: A Church-Dividing Issue? Its conclusion was direct: the Filioque is “no longer a ‘Church-dividing’ issue.”
The consultation, representing top theological authorities from both traditions, established several key findings. First, when both sides define their terms carefully, the apparent contradiction largely dissolves. The Latin Filioque — understood as affirming that the Spirit proceeds from Father and Son as from a single principle, with the Father as the fontal source — is compatible with the Eastern dia tou Huiou tradition defended by Maximus and others. Second, both traditions affirm the monarchy of the Father as the ultimate source within the Trinity. Third, the real problem is not the theological content but the unilateral insertion into the Creed.
“The problem [of the Filioque] is more in the area of semantics than in any basic doctrinal differences. The question of the Filioque should not be an insuperable obstacle to the reunion of Christendom.”
Ware is perhaps the most widely read Orthodox theologian writing in English. His frank concession that the problem is largely semantic rather than dogmatic is significant precisely because he has no interest in minimizing the Orthodox case.
The CCC (1992), promulgated under John Paul II with Cardinal Ratzinger as its primary architect, deliberately uses language designed to be compatible with Eastern theology: “The Eastern tradition expresses this relationship between the Father and the Son, saying that the Holy Spirit proceeds from the Father through the Son. The Western tradition expresses it by saying that he proceeds from the Father and the Son (Filioque). These formulations are legitimate and complementary.”
Three Orthodox Counter-Responses — and the Catholic Answers
If the theological problem is largely resolved and the patristic evidence cuts against a clean East-West binary, why has reunion not happened? Three Orthodox counter-responses must be taken seriously. Catholic apologetics that does not engage them has not engaged the real challenge.
“The Florence formula is verbal sleight of hand. The Council’s claim that the Spirit proceeds ‘as from one principle’ is theologically clever, but the Latin liturgy still recites simply Filioque — full stop. The ‘one principle’ qualifier is a theological gloss invisible to the worshipping faithful. The Creed as actually prayed still teaches what the East has always rejected.”
This objection has real force, and Catholic responses have begun to take it seriously. The PCPCU’s 1995 clarification was specifically aimed at this concern — making explicit, in authoritative Catholic teaching, that the Latin Creed is to be understood in the sense Florence defined and that Maximus had articulated centuries earlier. The catechetical work of communicating this to ordinary Catholics is real and ongoing. But what the objection cannot show is that the Filioque taught in Catholic doctrine contradicts Eastern theology — only that the surface phrasing of the Latin Creed obscures a theology that is fundamentally compatible with the Eastern position when properly understood.
“Citing Maximus is anachronistic. He wrote in the seventh century, before the controversy hardened, before the actual insertion into the Roman liturgy, before the procedural violation had taken full effect. By the ninth century, when the Filioque was actually being recited at Rome, the situation had changed irrevocably. The seventh century cannot be used to justify what the ninth century did.”
The objection has partial force but cuts both ways. If Maximus’s defense is anachronistic because the controversy hardened later, then so is Photius’s condemnation — which depends on a hardening that itself postdates the patristic consensus. The question raised by Maximus is not whether his exact historical situation is replicable but whether the underlying theological argument is sound. He claimed the Latins were not asserting two causes in the Trinity. The 2003 consultation confirmed, with the agreement of Eastern theologians, that they are not. If Maximus was right about the theology, the procedural issue alone — however serious — is not a sufficient reason for permanent schism between communions that share valid sacraments and apostolic succession.
“Rome has not removed the Filioque from its liturgy. Whatever scholars on commissions agree about doctrinal compatibility, the original procedural violation remains in liturgical use. Until the Catholic Church restores the original 381 text in its Latin Mass, the offense itself is enacted weekly in every Western parish. Doctrinal clarification cannot reverse a liturgical fact still in place.”
This is the strongest remaining objection, and it deserves the most serious Catholic engagement. The procedural violation is real — Catholic scholars from Yves Congar to Joseph Ratzinger have acknowledged it. The Eastern Catholic Churches, fully Catholic and in communion with Rome, recite the Creed without the Filioque, demonstrating that orthodoxy and the original creedal text are entirely compatible. The path forward most theologians on both sides envision involves Rome’s eventual restoration of the original 381 text in Latin liturgical use, while continuing to affirm the Filioque as orthodox theology that the Western tradition uniquely emphasizes. Such a move would not be a doctrinal capitulation but a recovery of the proper authority of an ecumenical council — exactly what Pope Leo III tried to safeguard with his silver tablets at the tomb of St. Peter.
Mark of Ephesus and the Failure of Florence
One Orthodox objection has not been answered, and any honest treatment of the Filioque must confront it: the Council of Florence (1439) was a formal reunion council that did precisely what the 2003 consultation later confirmed — it defined the procession from Father and Son as “from one principle and by a single spiration,” in language designed to reconcile East and West. Every Eastern bishop present signed the union decree.
Every Eastern bishop except one.
“I have not signed, and I will not sign, even if it should cost me everything. The truth must not be sold, even for the unity of the Church.”
St. Mark of Ephesus — metropolitan, theologian, and now a saint canonized by the Orthodox Church — refused. He returned to Constantinople, denounced the union, and within a few years the Eastern bishops who had signed renounced their signatures. By 1453, Constantinople had fallen to the Ottomans, the political pressure that drove the union evaporated, and the Florentine reunion had effectively died on the vine. Mark’s lone refusal proved historically dispositive.
For Orthodox readers, this is not a footnote. Mark is venerated as a Pillar of Orthodoxy. His refusal carries the weight of a saint’s witness against what he regarded as a coerced and theologically defective settlement. Catholic apologetics that ignores Mark, or treats him as merely the obstinate exception to an otherwise successful council, has not engaged the Orthodox case as Orthodox theology actually holds it.
Three things must be said honestly here. First: Mark’s theological objections were substantive, not merely disciplinary. He argued that “from one principle” was a verbal formula concealing the very two-cause theology Photius had condemned, and that Latin theologians at the council were equivocating between Augustine’s principaliter and a stronger Western position they were not willing to publicly disavow. These arguments deserve engagement, and modern scholarship — including Siecienski’s Oxford study — has taken them seriously rather than dismissing them.
Second: the political pressure on the Eastern delegation at Florence was real. Constantinople was militarily desperate, the union was a precondition for Western military aid, and the Greek bishops were negotiating under duress. Modern Catholic scholarship does not pretend otherwise. Yves Congar, John Meyendorff, and Joseph Ratzinger all acknowledged the political shadow over Florence’s reception.
Third, and decisively: Catholics nonetheless regard Florence’s dogmatic content as binding because the council was canonically convoked, the Roman pontiff confirmed its decrees, and the doctrinal substance — tanquam ab uno principio — preserves the Father’s monarchy in a way the 2003 consultation explicitly affirmed as compatible with Eastern theology. The political failure of the union does not invalidate the theological clarification. The clarification stands and continues to ground modern ecumenical dialogue.
What this means for reunion: Mark’s objections cannot be dismissed, but they have been substantially answered. The path forward involves Catholics taking the Mark of Ephesus tradition seriously as a legitimate theological reading of Florence’s ambiguities — and Orthodox theologians taking seriously that the most authoritative modern Catholic teaching, in Dominus Iesus, the 1995 PCPCU clarification, and the 2003 NAOCTC consultation, has resolved those ambiguities in directions Mark himself would likely have recognized as compatible with Eastern theology.
What Remains: The Procedural Issue
If the theological problem is largely resolved, what genuine obstacle remains? The procedural one. The Orthodox position — stated clearly in the 2003 consultation and in numerous formal dialogue documents — is that Rome must remove the Filioque from the Creed as used in the liturgy, restoring the original text of 381 AD. This is not a request that Catholics abandon the Filioque as a theological proposition; it is a request that Catholics stop reciting it in the universal Creed, which was defined by an ecumenical council and cannot be altered by any single church.
Catholic responses to this have been notably open. Pope John Paul II recited the Nicene Creed without the Filioque on multiple occasions when worshipping with Eastern Christians. The joint declaration at the Great Jubilee of 2000 between John Paul II and Patriarch Bartholomew I was signed without the Filioque. The Pontifical Council for Promoting Christian Unity issued a document in 1995, The Greek and Latin Traditions Regarding the Procession of the Holy Spirit, which acknowledged that the original Creed without the Filioque is also a legitimate expression of Catholic faith and is rightfully used by Eastern Catholic Churches in their liturgies.
This is the live frontier of the Filioque question. Not whether the doctrine taught is true (it is, when properly understood). Not whether the Eastern formula “through the Son” is also true (it is). The remaining question is whether Rome will eventually restore the original Creed of 381 in Latin liturgical use — affirming the Filioque as orthodox theology while removing the unilateral textual addition from the universal Creed defined at Constantinople. Pope Leo III gestured toward exactly this distinction with his silver tablets in 809. The shape of any future reunion almost certainly depends on Rome’s willingness to walk through the door he opened.
The Spirit proceeds from the Father alone. Any other formula compromises the Father’s monarchy.
The Holy Spirit proceeds principally from the Father, and, by the Father’s timeless gift, in common from both.
The dogma of the procession from the Father alone is the sole dogmatic foundation of the separation of East and West.
The Filioque is to be understood, expounded, and prayed in such a way that the East feels itself properly heard.
The problem is more in the area of semantics than in any basic doctrinal differences.
There is no citable basis for the claim historically made by both sides — that the Greek Fathers explicitly supported or denied the later theologies.
Where We Stand Today
A millennium of bitter polemic, two failed councils of reunion (Lyon II in 1274 and Florence in 1439), and centuries of mutual anathematization have made the Filioque feel like an unbridgeable theological chasm. The honest scholarly verdict, after decades of patient ecumenical work on both sides, is that the chasm is mostly imaginary.
The procedural violation was real. Pope Leo III saw this. The 2003 consultation affirms it. And the path forward almost certainly involves Rome’s eventual willingness to restore the original Creed in Latin liturgical use, while affirming Filioque theology as the West’s distinct and orthodox emphasis.
But the doctrinal divergence the East has long suspected — the charge that Catholics teach two causes in the Trinity, that they collapse the Father’s monarchy, that they undermine the personhood of the Spirit — is not what Catholic doctrine actually says. The Catechism explicitly affirms the Father as ultimate source. Florence speaks of “one principle.” Maximus, in the seventh century, defended Latin theology in Greek terms. The strongest Orthodox theologian of the twentieth century was opposing a position the most authoritative Catholic teaching does not actually hold.
Of all the Catholic-Orthodox flashpoints, the Filioque is the most resolvable — and arguably the most important to resolve, because it has stood for so long as the symbolic theological grounds for division. If that foundation can be honestly addressed, what remains belongs to the harder territory of papal primacy, ecclesial authority, and historical wounds — territory the rest of Track Two will now begin to explore.
What would honest reunion actually require? Not a theological surrender by either side, but a mutual recognition of what each tradition has rightly preserved — and a willingness to give symbolic ground where it costs nothing essential.
- Restore the 381 Creed in Latin liturgical use. Not abandon Filioque theology, but recover the original ecumenical text as it was defined at Constantinople — following the precedent already established for Eastern Catholic Churches in communion with Rome.
- Honor Pope Leo III’s witness. Affirm publicly what his silver tablets at the tomb of St. Peter affirmed: that no single patriarchate has the authority to alter a creedal text defined by an ecumenical council, even when its theological content is correct.
- Take Mark of Ephesus seriously. Engage his theological objections to Florence honestly rather than treating his refusal as obstinacy. The Catholic Church can hold Florence’s decrees as binding while acknowledging that Mark’s critique surfaced real ambiguities the magisterium has subsequently clarified.
- The Catholic Filioque is not a two-causes theology. The 2003 NAOCTC consultation already affirmed this with the agreement of Orthodox theologians; that finding needs to be received and taught at the parish level, not just cited in academic settings.
- Maximus is part of the Orthodox tradition. A revered Eastern Father defended Latin theology in Greek terms two centuries before Photius condemned it. Orthodox theology has the resources to integrate Maximus’s defense without abandoning the Eastern formulation.
- Augustine is not the heresiarch Lossky implied. The principaliter language in De Trinitate preserves the Father’s monarchy. The deeper systematic critique of Latin essentialism is a real conversation, but it is distinct from the dogmatic charge that the Filioque teaches two causes.
The Filioque was a clause that cracked the Creed.
It need no longer be a clause that cracks the Church.
Works Cited
- Maximus the Confessor. Letter to Marinus the Cypriot Priest. In Andrew Louth, trans., Maximus the Confessor. London: Routledge, 1996, pp. 171–195.
- Photius of Constantinople. The Mystagogy of the Holy Spirit. Trans. Joseph P. Farrell. Brookline, MA: Holy Cross Orthodox Press, 1987.
- Augustine of Hippo. The Trinity (De Trinitate). Trans. Edmund Hill, OP. The Works of Saint Augustine, I/5. Hyde Park, NY: New City Press, 1991. Book XV, ch. 26–27.
- Mark of Ephesus. Encyclical Letter Against the Greco-Latins and the Decrees of the Council of Florence. In Constantine N. Tsirpanlis, Mark Eugenicus and the Council of Florence. Center for Byzantine Studies, 1979.
- Gill, Joseph. The Council of Florence. Cambridge: Cambridge University Press, 1959.
- Cyril of Alexandria. Thesaurus on the Holy and Consubstantial Trinity. PG 75.
- John of Damascus. An Exact Exposition of the Orthodox Faith. Trans. Frederic H. Chase, Jr. Fathers of the Church 37. Washington, DC: CUA Press, 1958.
- Catechism of the Catholic Church, 2nd ed. Vatican City: Libreria Editrice Vaticana, 1997. §§246–248.
- Lossky, Vladimir. The Mystical Theology of the Eastern Church. London: James Clarke & Co., 1944.
- Bulgakov, Sergius. The Comforter. Trans. Boris Jakim. Grand Rapids: Eerdmans, 2004.
- Siecienski, A. Edward. The Filioque: History of a Doctrinal Controversy. Oxford: Oxford University Press, 2010.
- North American Orthodox-Catholic Theological Consultation. The Filioque: A Church-Dividing Issue? An Agreed Statement. Washington, DC: USCCB, October 25, 2003.
- Pontifical Council for Promoting Christian Unity. The Greek and Latin Traditions Regarding the Procession of the Holy Spirit. L’Osservatore Romano (English edition), September 13, 1995.
- John Paul II. Ut Unum Sint: On Commitment to Ecumenism. Encyclical Letter, May 25, 1995.
- Ratzinger, Joseph. Principles of Catholic Theology: Building Stones for a Fundamental Theology. Trans. Sister Mary Frances McCarthy, SND. San Francisco: Ignatius Press, 1987.
- Ware, Kallistos. The Orthodox Church. Rev. ed. London: Penguin Books, 1993.