Edict of Thessalonica
The Edict of Thessalonica, issued on 27 February AD 380 by Theodosius I, made Nicene Christianity the state church of the Roman Empire. It condemned other Christian creeds such as Arianism as heresies of "foolish madmen", and authorized their punishment.
This edict, addressed to the inhabitants of Constantinople whom Theodosius wished to pacify in order to make the city his imperial residence, constitutes the first known secular law which includes in its preamble a clear definition of what a Christian Roman ruler considers as religious orthodoxy, opening the way of repression against dissidents qualified as "heretics". The Edict of Thessalonica was subsequently incorporated into Book XVI of the Theodosian Code and was the milestone of the official Christianization of the Roman Empire.
Background
In 313 the emperor Constantine I, together with his eastern counterpart Licinius, issued the Edict of Milan, which granted religious toleration and freedom for persecuted Christians. By 325 Arianism, a school of christology which contended that Christ did not possess the divine essence of the Father but was rather a primordial creation and an entity subordinate to God, had become sufficiently widespread and controversial in Early Christianity that Constantine called the Council of Nicaea in an attempt to end the controversy by establishing an empire-wide, i.e., "ecumenical" orthodoxy. The council produced the Nicene Creed#Original [Nicene Creed of 325|original text] of the Nicene Creed, which rejected the Arian confession and upheld that Christ is "true God" and "of one essence with the Father."However, the strife within the Church did not end with Nicaea, and the Nicene credal formulation remained contentious even among anti-Arian churchmen. Constantine, while urging tolerance, began to think that he had come down on the wrong side, and that the Nicenes—with their fervid, reciprocal persecution of Arians—were actually perpetuating strife within the Church. Constantine was not baptized until he was near death, choosing a bishop moderately sympathetic to Arius, Eusebius of Nicomedia, to perform the baptism.
Constantius II, Constantine's son and successor in the eastern empire, was partial to the Arian party, and even exiled pro-Nicene bishops. Constantius' successor Julian was the only emperor after the conversion of Constantine to reject Christianity, attempting to fragment the Church and erode its influence by encouraging a revival of religious diversity, calling himself a "Hellene" and supporting forms of Hellenistic religion. He championed the traditional religious cultus of Rome as well as Judaism, and furthermore declared toleration for all the various unorthodox Christian sects and schismatic movements. Julian's successor Jovian, a Christian, reigned for only eight months and never entered the city of Constantinople. He was succeeded in the east by Valens, an Arian.
By 379, when Valens was succeeded by Theodosius I, Arianism was widespread in the eastern half of the Empire, while the west had remained steadfastly Nicene. Theodosius, who had been born in Hispania, was himself a Nicene Christian and very devout. In August, his western counterpart Gratian promoted persecution of heretics in the west.