Edict of Thessalonica

The Edict of Thessalonica (also known as Cunctos populos), issued on 27 February AD 380 by three reigning Roman Emperors, made Nicene Christianity the state religion of the Roman Empire. It condemned other Christian creeds such as Arianism as heresies of madmen, and authorized their persecution.
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 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 (337), choosing a bishop moderately sympathetic to Arius, Eusebius of Nicomedia, to perform the baptism.
Constantine's son and successor in the eastern empire, Constantius II was partial to the Arian party, and even exiled pro-Nicene bishops. Constantius' successor Julian (later called "The Apostate") 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.
Edict.
The Edict of Thessalonica was jointly issued by Theodosius I, Gratian, and Valentinian II on 27 February 380. The edict came after Theodosius had been baptized by the bishop Ascholius of Thessalonica upon suffering a severe illness in Thessalonica.
Importance.
The edict was issued under the influence of Ascholius, and thus of Pope Damasus I, who had appointed him. It re-affirmed a single expression of the Apostolic Faith as legitimate in the Roman Empire, "catholic" (that is, universal) and "orthodox" (that is, correct in teaching).
After the edict, Theodosius spent a great deal of energy trying to suppress all non-Nicene forms of Christianity, especially Arianism, and in establishing Nicene orthodoxy throughout his realm.
The edict was followed in 381 by the First Council of Constantinople, which affirmed the Nicene Symbolum and gave final form to the Nicene-Constantinopolitan Creed. In 383, the Emperor ordered the various non-Nicene sects (Arians, Anomoeans, Macedonians, and Novatians) to submit written creeds to him, which he prayerfully reviewed and then burned, save for that of the Novatians. The other sects lost the right to meet, ordain priests, or spread their beliefs. Theodosius forbade heretics to reside within Constantinople, and in 392 and 394 confiscated their places of worship.