While these political and dynastic changes had supplied the Empire with a new ruler almost every year, the growing alienation between East and West had been marked decisively by the separation of the Churches. Two great names embody in the East the final protest against Roman assumption. The Church of Constantinople had never abandoned its claim to equality with that of Rome, though it allowed to the ancient city the primacy of honour. Photius, who became patriarch in 858, and died in 891, owed his throne to an election which was not canonical, and though a council in 861 at Constantinople, at which papal legates were present, confirmed him in his office, Pope Nicholas I. declared that its decisions were illegal, and that Photius was deposed and excommunicated, while the Emperor himself was attacked in language of peculiar vehemence. The papal claim to decide between two claimants to the patriarchate was fiercely resented. Photius declared the equality of his see with that of Rome. To the Roman claim of jurisdiction, complicated also by assertions of supremacy over the Bulgarian Church, were added points of theological contention which the churches debated with as much eagerness, and it would seem, as little desire, to arrive at a reasonable solution. The addition of the words _Filioque_ to the Nicene Creed, asserting the Procession of the Holy Ghost from the Father and the Son, was, and is, resented by the Greeks as an addition to "the faith once for all delivered to the Saints." The use of unleavened bread in the Holy Eucharist was regarded in the East as an heretical innovation. There were, and are, other points of dispute; but none, it is probable, but for the strong national feeling of Italy and of Greece, would have caused a final breach.
The position which Photius defended with skill and vigour in the ninth century was reasserted by Michael Cerularius in the eleventh. He regarded the teaching of the West on the doctrine of the Holy Trinity, says Psellus, as an intolerable heresy; and he was prompt to reassert jurisdiction over the churches of Apulia, now conquered by the Normans and made subject to Rome. The final breach came from Rome itself. On July 16, 1054, two legates of the Pope laid on the altar of S. Sophia the act of excommunication which severed the patriarch from the communion of the West, and condemned what were asserted to be seven deadly heresies of the Eastern Church.