The Edict of Thessalonica, also known as Cunctos populos, was issued on 27 February 380 AD. It ordered all subjects of the Roman Empire to profess the faith of the bishops of Rome and Alexandria, making Nicene Christianity the state religion of the Roman Empire.
The emperor Constantine I converted to Christianity in 312. By 325 Arianism, a type of christology that contended that Christ was created and a subordinate entity to God the Father, had become so sufficiently popular and controversial in Early Christianity that Constantine called the Council of Nicaea in an attempt to end the controversy by establishing an empire-wide orthodoxy. The council produced the original Nicene Creed, which rejected Arianism 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. Constantine, while urging tolerance, began to think that he had come down on the wrong side, and that the Nicenes — with their fervid persecution of Arians — were actually perpetuating strife within the Church. Constantine was not baptised until he was near death (337), choosing an Arian bishop, Eusebius of Nicomedia, to perform the baptism.
Constantine’s son and successor in the east, Constantius II was sympathetic to the Arians, and even exiled Nicene bishops. Constantius’ successor Julian was the only emperor after the conversion of Constantine to reject Christianity and to attempt a revival of religious diversity, calling himself a “Hellene” and supporting forms of Hellenistic religion, the traditional religious cultus of Rome, and Judaism, as well as declaring toleration for all the various Christian sects. Julian’s successor, Jovian, a Christian, reigned for only eight months and never entered 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 part of the Empire, while the west had remained staunchly Nicene. Theodosius, who had been born in Hispania, was himself a Nicene Christian and very devout. In August, his counterpart in the west, Gratian, promoted persecution of heretics in the west.
The edict was issued under the influence of Acholius, 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 suppressing 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.