Abstract
This chapter discusses Constantine’s conversion and the transformation of Christianity into an “imperial church”, the church councils and doctrinal developments of the period, and important trends in Christian life. During the course of the fourth century, Christianity underwent a great reversal in its relationship with the Roman Empire. Modern historians have argued about whether Constantine was really a Christian. Constantine inspired the growth of a Christian devotion to the Roman state as an institution willed by God. The most significant theological achievement of the fourth and fifth centuries is the formal defining of the doctrines of the Trinity and the Incarnation, which are complementary Christian teachings about God and Christ that distinguish Christianity from all other religions. The Christianization of the Roman state brought the government into theological debate: disagreement and disunion had legal and political consequences, because rulers believed that divine protection of the empire depended on unity in the Church.