|Edict of Milan|
Emperor Diocletian pursued a comprehensive program against Christianity from 302 c.e. until his retirement in 305 c.e. His successors continued hostilities toward the church, especially in the eastern empire for several years, until it became clear that such programs were futile.
Sometime around 311 Galerius, one of the ruling Caesars, grudgingly and condescendingly issued the Edict of Toleration for all religious subjects, understood to apply mainly to the benefit of the persecuted Christians.
Shortly thereafter Galerius died. The western empire’s Caesar, Constantine the Great, immediately seized initiative and forged a similar agreement at Milan in 313 with his eastern counterpart Licinius.
This edict was more sympathetic to the Christian cause, reflecting Constantine’s sympathies for the faith. In time Christian causes even started to receive funds from the imperial treasury.
Ten years later Licinius unsuccessfully broke from Constantine’s religious revolution and renounced the accord of Milan; some 40 years later Constantine’s nephew Julian the Apostate also went this route and tried to reinstate conventional Greco-Roman religion.
The chronology and development of the Edict of Toleration and Edict of Milan is suspect, as the main sources (Lactantius and Eusebius of Caesaria) do not agree in detail; nonetheless, it is clear that Christians won their civic rights through these proclamations.
Contrary to popular opinion, Constantine the Great did not make Christianity the official religion of the Roman Empire. Only overt and widespread persecutions stopped. In fact, it was Theodosius I, called "the Great" by an appreciative church, who issued the edict Cunctos Populos in 380 that made orthodox teachings on the Trinity and the Incarnation of Jesus (Christ) of Nazareth mandatory for all citizens.
Anyone who did not go along was deemed "an extravagant madman". In 381 he summoned the bishops to the Council of Constantinople, as he began to deal seriously with church divisions.
Ten years later he fined and forcibly removed all church leaders who accepted Arianism. In addition, he forbade all Roman officials from participating in Greco-Roman religious sacrifices. By 392 Theodosius I had banned all pagan worship.
These aggressive religious programs effectively established Christianity as the state religion. From the fourth century onward Orthodox or Catholic Christianity was the dominant religion in the Mediterranean world.