Three great splits and none of them was purely doctrinal. 451: Chalcedon's formula on the two natures of Christ was rejected by the Egyptian, Syrian, Armenian and Ethiopian churches, with imperial coercion and regional resentment doing much of the work; those churches are the Oriental Orthodox, and modern dialogue has concluded the christological difference was largely terminological. 1054: legates from Rome laid a bull of excommunication on the altar of Hagia Sophia, the patriarch responded in kind, and the accumulated grievances over papal authority and the filioque clause hardened into permanent separation — the sack of Constantinople by crusaders in 1204 did far more damage than the bull. 1517: Luther's theses opened a rupture across the West that produced thousands of denominations and a century of religious war. The 1054 excommunications were mutually lifted by Paul VI and Athenagoras in 1965, and Rome and Constantinople still do not share communion, which is a good measure of how little formal gestures settle.