Clear the ground first, because the honest answer surprises people on both sides. The Catholic Church does not regard the Orthodox as Protestants of the East, nor as a body that lost the faith. She calls them true particular Churches — possessing valid bishops, a valid priesthood, a true Eucharist, and an unbroken apostolic succession. What Catholics and Orthodox share is nearly everything: seven sacraments, the Real Presence, the veneration of Mary as Theotokos and of the saints, sacred Tradition, and a thousand years of common councils and common Fathers.

So the division is narrow — but it is real, and it is not a mere misunderstanding. Two issues are genuinely church-dividing. The first is the papacy: the Catholic dogma that the Bishop of Rome holds a universal, immediate jurisdiction over the whole Church and can, in defined conditions, teach infallibly — which the Orthodox read as overturning the conciliar order of the first millennium, where the faith was guarded by the whole episcopate gathered in council. The second is the Filioque — the Western addition of “and the Son” to the Creed, objected to on both doctrinal and procedural grounds.

Almost everything else is secondary — the Immaculate Conception, the framing of purgatory — and even the weightier matters, such as St. Gregory Palamas’s essence–energies distinction (which the Orthodox themselves rank as dogma, not a footnote), are not what first broke communion. These are real differences, some of them grave; but they are not the hinge. The Catechism itself measures the remaining distance with startling precision: with the Orthodox the communion is so deep that it “lacks little to attain the fullness that would permit a common celebration of the Lord’s Eucharist.”2

Honest men should name the hard knot plainly. The Filioque, for all its bitter history, has been brought close to resolution: Rome and the East now largely agree on the doctrine beneath the word, as we will see. What has not been resolved is the papacy. When Rome states why full communion is still lacking, she does not point to the sacraments, or to Mary, or to the Fathers. She points to one thing: the Orthodox “do not accept the Catholic doctrine of the Primacy.” That is the dividing line — the one outstanding dogmatic obstacle of the first rank. The schism is a wound in the one Body of Christ, not a wall between two religions.