The Creed of 381 confessed the Holy Spirit “who proceeds from the Father.” Centuries later the Latin West sang it with one word added: Filioque — “and the Son.” That word is the single most-cited cause of the East–West schism. But the Orthodox objection has two prongs, and the Catholic who blurs them will never answer it.

The doctrinal prong. The Greek Fathers guard the monarchia of the Father: the Father alone is the one unoriginate Source — of the Son by begetting, of the Spirit by procession. To say the Spirit proceeds “from the Father and the Son” sounds, to Eastern ears, like smuggling a second source into God, or (in Photius’ sharp formula) like denying that the Spirit proceeds “from the Father alone.”

The Catholic answer to the doctrine. Rightly understood, the Filioque never made the Son a second cause. St. Maximus the Confessor — himself a Greek — defended the Latins in the seventh century: they “do not make the Son the cause of the Spirit,” he wrote, but mean that the Spirit proceeds from the Father through the Son. The Council of Florence later defined the procession as “from one principle and a single spiration,” not two. And in 1995 Rome herself clarified that the Father is “the sole Trinitarian Cause” of the Spirit — conceding the very point the East defends. So understood, Catholics argue, the remaining gap is largely one of grammar (Greek ekporeusis against Latin processio) rather than of faith. Most Orthodox do not grant that the question dissolves so neatly — for them the Filioque still touches the inner balance of the Trinity — and the one council that defined union on these terms, Florence, was repudiated in the East, St. Mark of Ephesus alone refusing to sign. The doctrinal distance may be narrower than the polemics suggest; it is not yet nothing.

The procedural prong — and the honest concession. Here the East has the stronger case, and Catholics should grant it cleanly. The Creed is the common property of the whole Church, and the West inserted Filioque into it unilaterally, without an ecumenical council. Rome herself long resisted the change: when Frankish enthusiasm pressed it, Pope Leo III affirmed the doctrine yet had the Creed engraved at St. Peter’s in Greek and Latin without the addition, to protect the original text. The doctrine can be defended; the manner of its insertion was a real wound — and saying so plainly is where any honest reunion begins.