The history of the Byzantine religion starts in 325 A.D. when Constantine I, the Roman emperor, declared Christianity the official Roman religion and chose Byzantium to be the head of the religion in 330 A.D. The emperor renamed the city Constantinople after himself. Before this, Byzantium was a Greek city established in the seventh century B.C.
Although Rome fell in 476 A.D., the eastern half of the Roman Empire, or the Byzantine Empire, thrived. Byzantium's Council of Chalcedon divided the established Christian religion into five regions or patriarchates: Constantinople, Alexandria, Jerusalem, Antioch and Rome. Moreover, the Byzantine emperor made himself the leader of both the empire and the church.
Justinian I, Byzantine emperor from 527 to 565 B.C., made great strides, including the building of the Church of Holy Wisdom and expansion into the former Western Roman Empire. However, taxes and outside attacks weakened the empire, and three of the patriarchates -- Alexandria, Antioch and Jerusalem -- fell to the Muslims in the seventh century A.D. Constantinople became the center of Christianity in the East.
Both the Christians ruled by the pope in Rome in the west and the eastern Christians in the Byzantine Empire went to war against the Muslims, a holy war that lasted 200 years from 1095 to 1291 A.D. However, the Crusades ultimately failed to regain the Holy Land, and the Ottomans took over the Hagia Sophia, the Church of Holy Wisdom. It was turned into a Muslim mosque, and the Byzantine Empire fell.