By the late third century most Romans tolerated Christianity, even if they did not practice it. Constantine ordered toleration of all religions in the Edict of Milan, issued in 313. He supported the church throughout his reign, expecting in return the support of church officials in maintaining order, and late in his life he was baptized as a Christian. Constantine also freed the clergy from imperial taxation and endowed the building of Christian churches.

Helped in part by its favored position in the empire, Christianity slowly became the leading religion, and emperors after Constantine continued to promote it. In 380 the emperor Theodosius (r. 379–395) made Christianity the official religion of the empire. He allowed the church to establish its own courts and to use its own body of law, called “canon law.” With this he laid the foundation for later growth in church power (see Chapter 8).