Religious life in the Roman Empire of the first four centuries carried a destructive contradiction: cult plurality was broadly tolerated, but Christians, who refused to take part in the imperial cult — and were therefore deemed politically disloyal — faced sporadic but heavy persecution. The Great Persecution under Diocletian (303–311) was the last and harshest. Galerius's deathbed edict in 311 officially ended persecution; in 313 the joint decisions taken at Mediolanum (modern Milan) by Constantine and Licinius set toleration as a durable legal framework for the whole empire. The document — known as the 'Edict of Milan,' though it was more a set of joint instructions than a single formal text — granted Christians freedom of worship, the return of confiscated property, and equal legal standing with other religions.

The personal narrative is famous: the tradition that, before the battle of the Milvian Bridge in 312, Constantine saw a sign in the sky is reported by later Christian sources as the start of his conversion. Whether his choice was primarily political or religious, and how much was strategic, remains a debated historiographical question. Either way, Constantine placed himself as protector of the Christian Church: he intervened in episcopal disputes, convened the Council of Nicaea in 325, and built major churches at Jerusalem and Rome.

The Milan decisions are a turning point on three axes. Legally, systematic state pressure on Christianity ended. Politically, a partnership between church and state was laid down that would shape Byzantium and medieval Europe. Sociologically, an underground minority entered the path that would, within a generation, make it the empire's main religion. Theodosius's declaration of Christianity as the state religion in 380 is the logical conclusion of what Milan began.