The transformation of Roman religion from polytheistic civic cult to Christian state religion over the fourth century AD is one of the most consequential shifts in Western history. Diocletian's Great Persecution (303-311 AD) was the most systematic attempt to eradicate Christianity and the last. Constantine's Edict of Milan in 313 legalized Christian worship. By Theodosius I's reign (379-395), paganism was being actively suppressed.

The more interesting question than Constantine's sincerity is structural: why did Christianity prove so much more adaptable to imperial co-optation than competing cults like Mithraism or the Isis cult? Christianity had a clergy, a scriptural canon, episcopal organization, and a tradition of theological argument that gave it institutional coherence. It could be directed by councils and synods in ways a mystery cult could not.

The process was not clean or linear. Pagan practices survived in rural areas for centuries. Julian's brief pagan revival showed that paganism still had elite support as late as 361. But the structural momentum was irreversible after Constantine.