Constantine I (reigned 306–37) adopted Christianity as his system of belief after the Battle of Milvian Bridge in 312. His victorious legions fought under the "labarum", a standard with the first two Greek letters of Christ's name (XP).
In 313 the Edict of Milan legalised Christianity alongside other religions allowed in the Roman Empire. In 325 the First Council of Nicaea signalled consolidation of Christianity under an orthodoxy endorsed by Constantine, and though this did not make other Christian groups outside the adopted definition illegal, the dissenting Arian bishops were initially exiled. But Constantine reinstated Arius just before the heresiarch died in 336 and exiled the Orthodox Athanasius of Alexandria from 335 to 337. In 380 Emperor Theodosius I made Christianity the Roman Empire's official religion (see State church of the Roman Empire, Byzantine Empire and the Goths) and did enforce the edict. In 392 Theodosius passed legislation prohibiting all pagan cultic worship.
During the 4th century, however, there was no real unity between church and state: in the course of the Arian controversy, Arian or semi-Arian emperors exiled leading Trinitarian bishops, such as Athanasius (335, 339, 356, 362, 365) Hilary of Poitiers (356), and Gregory of Nyssa (374); just as leading Arian and Anomoean theologians such as Aëtius (fl. 350) also suffered exile.
Towards the end of the century, Bishop Ambrose of Milan made the powerful Emperor Theodosius I (reigned 379–95) do penance for several months after the massacre of Thessalonica (390) before admitting him again to the Eucharist. On the other hand, only a few years later, Chrysostom, who as bishop of Constantinople criticized the excesses of the royal court, was eventually banished (403) and died (407) while traveling to his place of exile.