This series began with a question about papal primacy and ends with a question about whether the two oldest Christian traditions can find their way back to each other. The six preceding articles have examined the major disputed questions one by one—primacy and the pentarchy, doctrinal development, the Filioque, papal infallibility, the Marian dogmas, the theology of prayer—and the picture that emerges is more nuanced and more hopeful than either side’s popular polemic usually admits. The barriers are real. Some are tall. But none is infinite, and several are shorter than a millennium of mutual recrimination has made them appear.

This capstone article is different from the others. It is not an argument for a Catholic position against an Orthodox one. It is a map—an honest assessment of where the dialogue stands, what has been achieved, what remains, and what reunion would actually look like in practice. The theology is not hypothetical. Real documents exist. Real proposals sit on the table. The question is no longer whether reunion is theologically possible. The six preceding articles have shown that it is. The question is whether the two traditions have the will to act on what they already know.

I. What Has Already Been Agreed

The progress of the last half-century is easy to underestimate because it has been slow, technical, and largely invisible to the faithful in the pews. But the Joint International Commission for Theological Dialogue, established at Rhodes in 1980, has produced a series of agreed statements that represent genuine and unprecedented convergence. These are not Catholic documents or Orthodox documents; they are joint documents, unanimously approved by both delegations.

The most fundamental agreement is sacramental. Both churches recognize each other’s sacraments as valid. Both have apostolic succession. Both celebrate the same Eucharist, even if they do not currently share it. This is not a minor point—it means that the two traditions are not starting from scratch. The foundation of reunion already exists in the sacramental reality that both churches share. A Catholic who walks into an Orthodox liturgy is witnessing a valid Eucharist; an Orthodox who walks into a Catholic Mass is witnessing the same. The separation is real, but it is a separation within a shared sacramental life, not a separation between two religions.

The Ravenna Document (2007) achieved what decades of earlier dialogue had prepared: a joint statement on the relationship between primacy and conciliarity. Both sides agreed that the Church requires both—that primacy without conciliarity becomes tyranny, and conciliarity without primacy becomes chaos. Both agreed that this interdependence operates at every level of the Church’s life: local (the bishop in his diocese, with his clergy), regional (the metropolitan or patriarch, with his synod), and universal. And both agreed on the crucial point that had been contested for a millennium: there has always been a protos at the universal level.

The significance of Ravenna is not that it resolved the primacy question—it did not. It is that both sides agreed there is a question to resolve. The Orthodox had long argued that there is no universal primacy, only regional primates who are all equal. Ravenna moved past that position: there has always been a protos. The question that remains is not whether the bishop of Rome has a special role at the universal level but what that role is—what the protos is authorized to do.

The Chieti Document (2016) took the next step, examining how primacy and synodality actually functioned in the first millennium. It surveyed the historical evidence: the canons of the early councils, the appeals to Rome, the role of the pope in convoking and confirming councils, and the ways in which Roman primacy was exercised differently in the East and the West even while both remained in communion. Chieti did not resolve the question either, but it provided the historical data that any resolution must reckon with—and it showed that the first millennium itself was more complex than either side’s narrative usually admits.

II. The Ratzinger Formula

The most consequential sentence in modern Catholic ecumenism was spoken by a young German theologian at an ecumenical gathering in Graz, Austria, in 1976:

The full formula is more demanding than the famous first sentence suggests. Ratzinger continued: “Reunion could take place in this context if, on the one hand, the East would cease to oppose as heretical the developments that took place in the West in the second millennium and would accept the Catholic Church as legitimate and orthodox in the form she had acquired in the course of that development, while, on the other hand, the West would recognize the Church of the East as orthodox and legitimate in the form she has always had.”

This is not a surrender by either side. It is a framework for unity in diversity. The formula asks two things, one of each tradition:

The brilliance of the formula is that it does not ask either side to say it was wrong. It asks each side to say the other was right in its own way. The West would continue to believe in papal infallibility; the East would not be required to share that belief, only to stop calling it heresy. The East would continue to reject the Filioque in its Creed; the West would not require its adoption, only its toleration. Both would be fully themselves, in full communion, without pretending to agree on everything.

The formula’s symmetry, however, is more apparent than real—and honesty requires naming this. What does the West actually give up? In practice, nothing. The pope remains pope, Vatican I remains Vatican I, the Marian dogmas remain Catholic doctrines. The West simply agrees not to impose these on the East—which is already the status quo. The East, by contrast, is asked to do something genuinely costly: accept Vatican I as “legitimate and orthodox,” even while not accepting it as binding. That means the East would need to say, in effect: “Papal infallibility is a legitimate development of Christian doctrine, even though we reject it for ourselves.” Many Orthodox cannot say this. It would feel like endorsing a theological claim they believe is wrong—not merely tolerating it but calling it legitimate. The formula asks more of the East psychologically and theologically than it asks of the West, and this asymmetry is one reason it has not been accepted. Any future negotiation will need to address this imbalance directly.

The formula has complications that must be named honestly. First, Ratzinger articulated it as a private theologian in 1976, not as pope. When he became Benedict XVI, he did not formally promulgate it as papal teaching, though he continued to speak of the first millennium as a model. Second, a 1998 symposium organized by the Congregation for the Doctrine of the Faith raised questions about whether the first millennium itself supports the formula as neatly as it suggests—Roman primacy in the first millennium was already more than a primacy of honor in some readings. Third, some Orthodox theologians have rejected the formula as still asking too much: accepting Vatican I as “legitimate,” even if not binding, is a concession they are unwilling to make.

These complications are real but they do not invalidate the formula. They reveal that the formula is a starting point for negotiation, not a finished agreement—and that is exactly what it was intended to be.

III. The Real Obstacles — Theological

Not all disagreements are equally difficult. The six preceding articles have sorted them, and this section arranges them by height—from the shortest barriers to the tallest.

The Filioque (resolvable). Article III examined this in detail. The Catholic Church has already acknowledged that the Creed should be recited without the Filioque when both traditions worship together. Theological formulas exist—“from the Father through the Son,” which Maximus the Confessor endorsed in the seventh century—that both sides can affirm. Of all the theological obstacles, this is the most solvable. It is a dispute about a word, not about the reality the word describes.

The Assumption/Dormition (narrow gap). Article V demonstrated that both traditions believe the same thing: Mary is bodily in heaven. The Orthodox celebrate it liturgically as the Dormition; the Catholics defined it dogmatically as the Assumption. Under the Ratzinger Formula, the definition would remain a Catholic doctrine not binding on the East. The gap is almost entirely about authority, not content.

The essence/energies distinction (theological pluralism). Article VI touched this. The Catholic theologian G. Philips called it “a perfectly admissible theological pluralism.” Several Catholic and Orthodox scholars have argued it is compatible with Catholic theology. It is a genuine metaphysical question, but it is a question among specialists, not a barrier between churches.

The Immaculate Conception (deeper gap). Article V showed that this rests on the original sin/ancestral sin divergence—a soteriological question, not merely a Mariological one. Under the Ratzinger Formula, the IC would remain a Catholic doctrine not binding on the East. But the underlying soteriological divergence would remain genuinely unresolved.

Doctrinal development (framework question). Article II examined this. The Orthodox resistance to the concept of development is the meta-obstacle: not a specific disagreement but a difference in how the two traditions handle disagreement. Under reunion, the East would not need to accept development as the West understands it—only to accept that the West’s developments are “legitimate and orthodox.”

Papal jurisdiction (the tallest wall). Article I examined this, and Article IV traced its formal definition. This is the central question. The Orthodox can accept a primacy of honor. They cannot accept a primacy of universal jurisdiction as defined by Vatican I. The Ratzinger Formula offers a path: the pope would exercise jurisdiction over the West as he does now, and exercise a different form of primacy over the East—a primacy of honor, of appeal, of convening councils, but not of direct governance.

But this raises a question that must be faced squarely: is the Ratzinger Formula compatible with Pastor Aeternus? Vatican I defines the pope’s jurisdiction as “truly episcopal” and “immediate” over “each and all of the churches, and each and all of the pastors and the faithful.” If this jurisdiction is universal by divine institution, can the pope simply choose not to exercise it over the East? The answer requires a distinction that Vatican II’s Lumen Gentium already began to develop: between the substance of an office and the form of its exercise. The CDF’s 1998 symposium on the primacy of the successor of Peter acknowledged that “the concrete forms of the exercise of the Petrine ministry” have varied throughout history and can vary again. The pope’s jurisdiction is real, but its exercise is historically conditioned—and the first millennium provides a model of how it can be exercised differently in different ecclesial contexts without ceasing to exist. This is not a reinterpretation of Vatican I; it is a reading of Vatican I in the light of Vatican II’s more collegial ecclesiology. Whether the Orthodox will accept this distinction is an open question—but the Catholic resources for making it are genuine.

IV. The Real Obstacles — Beyond Theology

Not all obstacles are theological. Some are political, cultural, and psychological, and they are no less real for being non-doctrinal.

Autocephaly and nationalism. Orthodox ecclesiology is organized by nation: Greek, Russian, Serbian, Romanian, Bulgarian, Georgian. These national churches have their own patriarchs, their own politics, their own interests. Reunion with Rome would require all of them to agree—and they cannot always agree with each other. The granting of autocephaly to the Orthodox Church of Ukraine by Constantinople in 2019, over Moscow’s vehement objection, shattered the already fragile unity of the Orthodox world. A communion that cannot maintain internal consensus is not well positioned to negotiate external reunion.

Moscow. The Russian Orthodox Church, the largest Orthodox church by membership, has been the most resistant to ecumenical dialogue—partly for theological reasons and partly for political ones. The current geopolitical situation has made dialogue harder, not easier. The Moscow Patriarchate’s close relationship with the Russian state creates a dynamic in which theological positions are entangled with national interests in ways that make honest ecumenical engagement difficult.

The Eastern Catholic question. Twenty-three Eastern Catholic Churches already exist in communion with Rome while preserving Eastern liturgy, theology, and canon law. The Orthodox view them with deep suspicion—“Uniates” who abandoned Orthodoxy for Roman protection, often under political pressure. The Balamand Document (1993) acknowledged that Uniatism is not the model for reunion. But the Eastern Catholic Churches still exist, their faithful number in the millions, and their status in a reunited Church would need to be resolved. Would they retain their current canonical relationship with Rome? Would they be reintegrated into the Orthodox patriarchates from which they originally separated? Would they serve as bridges between the two traditions, or would their very existence remain a source of resentment? These questions have no agreed answers, and any honest proposal for reunion must address them rather than assume they will resolve themselves. The Eastern Catholic Churches are both a sign that East–West communion is possible and a reminder of how badly it can go wrong when reunion is imposed rather than received.

Psychological resistance. A millennium of separation has created identities defined partly by opposition. For many Orthodox, being Orthodox means not being Catholic. For many Catholics, the Orthodox are “almost Catholic but not quite.” These identities would need to expand, and identities do not expand easily. The theological obstacles may be smaller than they appear; the psychological obstacles may be larger.

V. What Reunion Would Actually Look Like

If the Ratzinger Formula were accepted as a framework and the Ravenna/Chieti process continued to its conclusion, what would a reunited Church actually look like? Not as a theological abstraction but as a lived reality?

The pope would remain pope. He would continue to exercise jurisdiction over the Western Church as defined by Vatican I and Vatican II. Nothing in the internal governance of the Catholic Church would need to change.

The pope would also be the protos of the reunited Church. At the universal level, he would hold a primacy modeled on the first millennium: first in honor, with specific prerogatives—convening ecumenical councils, hearing appeals, serving as a court of last resort in disputes between patriarchates—but without direct jurisdiction over the Eastern churches. This is not an innovation; it is a restoration of how the papacy actually functioned before the schism.

The Orthodox patriarchates would retain their autonomy. They would govern themselves as they do now, with their own liturgy, theology, canon law, and devotional culture. The patriarch of Constantinople would retain his title of “first among equals” within the Orthodox world. The internal structure of Orthodoxy would not need to change.

The Creed would be shared. In joint worship, the Creed would be recited without the Filioque—as is already the practice when pope and patriarch pray together. In the Latin Church’s own liturgy, the Filioque might be retained or dropped—a question of discipline, not doctrine, since the Catholic Church has already affirmed that the Creed can be legitimately recited both ways.

The Marian dogmas would remain Catholic doctrines. The East would not be required to accept them as binding. It would be required to refrain from condemning them as heresy—which, as Article V’s Lev Gillet citation demonstrated, the Orthodox have never formally done anyway.

Intercommunion would be restored. Catholics and Orthodox would share the Eucharist. This is not a small thing. It is, in many ways, the whole thing. The shared Eucharist is not a reward for theological agreement; it is the sacramental reality that makes the Church one. Its restoration would be the most visible and most consequential fruit of reunion.

Ecumenical councils could resume. A genuinely ecumenical council of the reunited Church—the first since Nicaea II in 787—could address the questions that have accumulated over a millennium. The very existence of such a council would be a theological event of the first order: the Church, speaking with one voice for the first time in a thousand years.

VI. Why It Hasn’t Happened Yet — And Why It Might

The theological groundwork has been laid. The Ravenna Document establishes the principle. The Chieti Document provides the historical data. The Ratzinger Formula offers the framework. John Paul II’s invitation in Ut Unum Sint signals the will. Why, then, has reunion not happened?

The answer is not primarily theological. It is political, cultural, and psychological. The Patriarch of Constantinople has consistently been open to dialogue; the Patriarch of Moscow has not. The Russian Orthodox Church’s entanglement with the Russian state makes theological dialogue inseparable from geopolitics—and the current geopolitical moment is not favorable. The Ukraine autocephaly crisis of 2019 fractured the Orthodox world in ways that make a unified Orthodox response to any proposal of reunion nearly impossible. You cannot negotiate reunion with a communion that is not at peace with itself.

Athenagoras’s words to Paul VI are remarkable because they express, in a single sentence, the essential content of first-millennium primacy as the Orthodox themselves understand it: first in honor, presider over charity. Ratzinger himself cited this moment as evidence that the formula already existed in practice before he articulated it in theory. If a patriarch can address a pope in these terms, the theological distance between the two traditions is not as great as a millennium of polemic has made it seem.

The honest assessment is this: reunion will not happen in the current generation. The political obstacles are too large, the psychological resistance too deep, and the internal divisions within Orthodoxy too acute. But the theological work of this generation—the dialogues, the documents, the formulas, the mutual visits, the lifting of excommunications, the shared prayers—is laying a foundation without which future reunion is impossible. The generation that achieves reunion will build on what this generation has prepared. The work matters even if its fruit is not yet visible.

VII. Counter-Responses