East–West Schism
In the East, Christendom became more defined as the Byzantine Empire's gradual loss of territory to an expanding Islam and the muslim conquest of Persia. This caused Christianity to become important to the Byzantine identity. Before the East–West Schism which divided the Church religiously, there had been the notion of a "universal Christendom" that included the East and the West. After the East–West Schism, hopes of regaining religious unity with the West were ended by the Fourth Crusade, when Crusaders conquered the Byzantine capital of Constantinople and hastened the decline of the Byzantine Empire on the path to its destruction. With the breakup of the Byzantine Empire into individual nations with nationalist Orthodox Churches, the term Christendom described Western Europe, Catholicism, Orthodox Byzantines, and other Eastern rites of the Church.