Believers in early Christian communities chose their own leaders, but over time appointment by existing church leaders or secular rulers became the common practice. During the reign of Diocletian (r. 284–305), the Roman Empire had been divided for administrative purposes into geographical units called dioceses, and Christianity adopted this pattern. Each diocese was headed by a bishop. The center of a bishop’s authority was his cathedral, a word deriving from the Latin cathedra, meaning “chair.”

The early Christian Church benefited from the administrative abilities of church leaders. Bishop Ambrose of Milan (339–397) was typical of the Roman aristocrats who held high public office, converted to Christianity, and subsequently became bishops. Like many bishops, Ambrose had a solid education in classical law and rhetoric, which he used to become an eloquent preacher. He had a strong sense of his authority and even successfully resisted Emperor Theodosius’s (r. 379–395) efforts to take control of church property. Ambrose’s assertion that the church was supreme in spiritual matters and the state in secular issues was to serve as the cornerstone of the church’s position on church-state relations for centuries.

Although conflicts between religious and secular leaders were frequent, the church also received support from the emperors. In 380 Theodosius made Christianity the official religion of the empire, and later in his reign he authorized the closure or destruction of temples and holy sites dedicated to the traditional Roman and Greek gods. In return for such support, the emperors expected the Christian Church’s assistance in maintaining order and unity.

Christians disagreed with one another about many issues. In the fourth and fifth centuries disputes arose over the nature of Christ. For example, Arianism, developed by Arius (ca. 250–336), a priest of Alexandria, held that Jesus was created by the will of God the Father and thus was not co-eternal with him. Emperor Constantine, who legalized Christianity in 312, rejected the Arian interpretation. In 325 he summoned a council of church leaders to Nicaea in Asia Minor and presided over it personally. The council produced the Nicene (nigh-SEEN) Creed, which defined the position that Christ is “eternally begotten of the Father” and of the same substance as the Father. Arius and those who refused to accept Nicene Christianity were banished. Their interpretation of the nature of Christ was declared a heresy, that is, a belief that contradicted the interpretation the church leaders declared was correct, which was termed orthodoxy. These actions did not end Arianism, however. Several later emperors were Arian Christian, and Arian missionaries converted many barbarian tribes. The Nicene interpretation eventually became the most widely held understanding of the nature of Christ.

Nonetheless, disputes about the nature of Christ also continued, with factions establishing themselves as separate Christian groups. The Nestorians, for example, regarded the divine and human natures in Jesus as distinct from one another, whereas the orthodox opinion was that they were united. The Nestorians split from the rest of the church in the fifth century after their position was outlawed and settled in Persia. Nestorian Christian missionaries later founded churches in Central Asia, India, and China (see Chapter 12).