From outlaws to the empire's faith: Constantine and legalization

For nearly three centuries Christianity was illegal or merely tolerated, and periodically it was violently persecuted, most severely under the emperors Decius (250) and Diocletian (the "Great Persecution," 303–311). The turning point came with Constantine, the first Roman emperor to favor the faith. After his victory at the Milvian Bridge (312), Constantine and his co-emperor Licinius issued the Edict of Milan in 313, granting Christians (and everyone else) freedom of worship and restoring confiscated property. Within a lifetime the church moved from the catacombs to the basilicas: Constantine funded churches, gave clergy legal privileges, and summoned the first empire-wide council. By 380, under Theodosius I, the Edict of Thessalonica made Nicene Christianity the official religion of the Roman state. Historians have long debated what this alliance of throne and altar cost the church, Schaff and many after him note that imperial favor brought wealth, status, and nominal converts, and entangled the church with state power in ways later reformers (and the martyrs' own memory) would question. But the fact is decisive: within one century Christianity went from a hunted minority to the established faith of the Roman world, and the institutional shape of the medieval church, bishops, dioceses, church councils backed by imperial authority, took form in that shift.