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.
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.
IMPPP. GR(ATI)ANUS, VAL(ENTINI)ANUS ET THE(O)D(OSIUS) AAA. EDICTUM AD POPULUM VRB(IS) CONSTANTINOP(OLITANAE).
Cunctos populos, quos clementiae nostrae regit temperamentum, in tali volumus religione versari, quam divinum Petrum apostolum tradidisse Romanis religio usque ad nunc ab ipso insinuata declarat quamque pontificem Damasum sequi claret et Petrum Aleksandriae episcopum virum apostolicae sanctitatis, hoc est, ut secundum apostolicam disciplinam evangelicamque doctrinam patris et filii et spiritus sancti unam deitatem sub pari maiestate et sub pia trinitate credamus. Hanc legem sequentes Christianorum catholicorum nomen iubemus amplecti, reliquos vero dementes vesanosque iudicantes haeretici dogmatis infamiam sustinere ‘nec conciliabula eorum ecclesiarum nomen accipere’, divina primum vindicta, post etiam motus nostri, quem ex caelesti arbitro sumpserimus, ultione plectendos.
DAT. III Kal. Mar. THESSAL(ONICAE) GR(ATI)ANO A. V ET THEOD(OSIO) A. I CONSS.
EMPERORS GRATIAN, VALENTINIAN AND THEODOSIUS AUGUSTI. EDICT TO THE PEOPLE OF CONSTANTINOPLE.
It is our desire that all the various nations which are subject to our Clemency and Moderation, should continue to profess that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition, and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria, a man of apostolic holiness. According to the apostolic teaching and the doctrine of the Gospel, let us believe in the one deity of the Father, the Son and the Holy Spirit, in equal majesty and in a holy Trinity. We authorize the followers of this law to assume the title of Catholic Christians; but as for the others, since, in our judgment they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give to their conventicles the name of churches. They will suffer in the first place the chastisement of the divine condemnation and in the second the punishment of our authority which in accordance with the will of Heaven we shall decide to inflict.
GIVEN IN THESSALONICA ON THE THIRD DAY FROM THE CALENDS OF MARCH, DURING THE FIFTH CONSULATE OF GRATIAN AUGUSTUS AND FIRST OF THEODOSIUS AUGUSTUS
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.
This Edict is the first which definitely introduces Catholic orthodoxy as the established religion of the Roman world. [...] Acknowledgment of the true doctrine of the Trinity is made the test of State recognition.