This structure carried the Church through wave after wave of heresies that warred against her unity. A major schism did eventually occur, however, largely due to political developments. From the time the Church was legalized, there had always been some sense of separation between the Patriarchate of Rome, which was Latin-speaking and associated with the Western Roman Empire, and the other four patriarchates, which were Greek-speaking and associated with the Eastern Roman Empire. From the fifth century on, this sense of separation was heightened as the Western Empire began to disintegrate, torn apart by barbarian invasions. The chaotic situation made communications with Constantinople problematic, as words could then be carried only by human beings, and travel through the war-torn regions between Rome and Constantinople was difficult if not impossible much of the time. Additionally, as institutions of learning were destroyed by the barbarian hordes, fewer and fewer people in the West could understand Greek. Theologians arose in the West (notably Augustine) who interpreted the Faith in ways that were subtly different from those of the rest of the Church; as these differences could not be corrected immediately, they grew into serious theological problems.

The most serious of these problems is referred to briefly as "the filioque." The Nicene Creed, adopted by the entire Church at the two Councils of Nicaea in the fourth century, stated, "I believe in the Holy Spirit . . . who proceeds from the Father, who is worshipped and glorified with the Father and the Son." For reasons too complicated to go into here, the West gradually came to adopt an alteration to the Creed, so that it read, "I believe in the Holy Spirit, who proceeds from the Father and the Son [in Latin, filioque] . . ." This addition has the effect of reducing the Holy Spirit to a secondary place in the Trinity, subordinate to the Father and the Son; it was soundly repudiated by the Church in the East.

Over the course of the following centuries, the Western Church, guided by Rome, grew gradually more distinct from and independent of the Eastern Church, and the bishops of Rome appropriated more and more power-both secular and ecclesiastical. Their division reached a climax in 1054, when the patriarchs of Rome and Constantinople mutually excommunicated each other. This event is referred to as the Great Schism. From this time on, the Church in the West called itself the Roman Catholic Church, while the Church in the East was known as the Eastern Orthodox Church ("Orthodox" meaning "right doctrine" or "right worship").

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