During the late Roman Empire, Emperor Constantine the Great radically changed the course of history by embracing the Christian religion. To provide some semblance of order, he also forced bishops across his empire to meet up and agree on what Christians actually believed at the First Council of Nicaea. In theory, Christians were supposed to have one faith and one set of doctrines.
However, as it turned out, the Roman Empire was not long for this world. The western half gradually collapsed, and the eastern half — known to us as the Byzantine Empire — kept on trucking for almost another 1000 years. In time, the beliefs and practices of those living in ex-Roman lands in the west and those living in Byzantium started to diverge quite naturally, and the complex politics of the Middle Ages exacerbated the split. The divide between the two groups came to a head in quite dramatic fashion in 1054, when a papal legate excommunicated the Patriarch of Constantinople by storming into the Cathedral of Hagia Sophia and placing a papal bull on the altar.
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The break was a long time coming. The Byzantines disagreed with the Western Church on everything from the celibacy of priests to the type of bread they should be used in Holy Communion. Most importantly, they refused to accept the pope in Rome as the undisputed head of the Church above their patriarch. The pope was unable to exert his authority in Byzantine lands, and the Eastern Orthodox Church was born as a result.
