There is exactly one first-rank, church-dividing obstacle: the papacy — specifically Vatican I’s definition of universal, ordinary, immediate jurisdiction and papal infallibility (Pastor Aeternus, 1870). Everything else is smaller, and intellectual honesty means saying so rather than flattening the inventory into a dozen equal grievances or waving the whole division away as a misunderstanding.

For the Orthodox the papacy is not one issue among many — it is the issue. They gladly grant Rome a primacy of honor, the senior see, the protos who “presides in love” — that is the first-millennium order they never abandoned. What they reject is the transformation of primacy into supremacy: a bishop with direct, immediate jurisdiction over every diocese on earth, who can define dogma irreformably of himself, and not by the consent of the Church. For the East, dogma is received by the whole Church and ratified by councils the body receives — not pronounced from a single chair. To accept Vatican I would be to cease being Orthodox; and Florence taught them that reunion bought by pressure does not hold.

Closely attached to the papacy is the ecclesiology of conciliarity versus juridical primacy — not really a separate question. The Filioque sits next, both the doctrine and the unilateral addition, largely clarified by dialogue but unresolved in practice. The genuinely secondary issues are real but bridgeable, and the dialogue has largely defused them: the Immaculate Conception (a difference about how ancestral sin is framed, not about Mary’s holiness), purgatory (a shared practice of prayer for the dead, differently defined), unleavened bread, and clerical celibacy — a discipline, not a dogma, which Rome already relaxes for her own Eastern Churches. And two non-theological wounds still poison the trust: the memory of 1204 and of Uniatism, and Orthodoxy’s own internal disunity (Moscow versus Constantinople), which leaves no one able to say “yes” for the whole East.

The honest Catholic answer refuses both temptations: the papacy is not a misunderstanding — it is a real dogma, defined de fide, which Rome cannot and will not retract — and it is not negotiable in substance. But its mode of exercise is open. John Paul II invited the Churches to help find “a way of exercising the primacy which… is nonetheless open to a new situation” (Ut Unum Sint §95); Ratzinger’s formula asked that Rome require of the East no more than the first millennium knew. The path is the one the dialogue has already taken at Ravenna and Chieti: recover how Rome’s primacy and the Church’s conciliarity actually coexisted before the schism — neither side surrendering its faith, both recovering a shared grammar. The dogma stands; the form is where the work is.