Edict of Thessalonica
The Edict of Thessalonica, also known as Cunctos populos, was an edict issued in 380 AD, that ordered all subjects of the Roman Empire to profess the faith of the bishops of Rome and Alexandria. The edict made Christianity the state religion of the Roman Empire.
The emperor Constantine I converted to Christianity in 312. By 325 Arianism, a type of christology which denied the Trinity, had created enough problems in Early Christianity that Constantine (who had little patience for the finer points of theology) called the Council of Nicaea in an attempt to establish an empire-wide orthodoxy and end the controversy. The council produced the original Nicene Creed, which rejected Arianism and upheld the Trinity.
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 trinitarians—with their fervent persecution of Arians—were actually perpetuating strife within the Church. Constantine was not baptized until he was near death ( 337), and then he chose 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 widespread revival of religious diversity, calling himself a "Hellene" and supporting forms of Hellenistic religion, the traditional religious cultus of Rome, and Judaism. His declared toleration for all the various Christian sects encouraged the less powerful in their disagreements.[vague] Julian's successor in turn, Jovian, while Christian, only reigned for 8 months and never entered Constantinople. He was then succeeded in the east by Valens who was 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, took steps toward legal persecution of heretics in the west.
IMPPP. GR(ATI)IANUS, 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 Acholius, and thus of Pope Damasus I, who had appointed him. It re-affirmed one expression of the Apostolic Faith as legitimate in the Roman Empire, "catholic" (that is, universal) and "orthodox" (that is, correct). 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 prohibited the residence of heretics within Constantinople, and in 392 and 394 confiscated their places of worship.
References and sources 
- Ehler, Sidney Zdeneck; Morrall, John B (1967). Church and State Through the Centuries: A Collection of Historic Documents with Commentaries. p. 6. ISBN 9780819601896.
- Williams & Friell, (1994) pp. 46–53
- Codex Theodosianus XVI.1.2
- "Theodosius I". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
- Boyd (1905), p. 45
- Boyd (1905), p. 47
- Boyd (1905), p. 50
- Boyd, William Kenneth (1905). The Ecclesiastical Edicts of the Theodosian Code, Columbia University Press
- Ferguson, Everett; McHugh, Michael P.; Norris, Frederick W. (1999). Encyclopedia of Early Christianity, Taylor & Francis ISBN 0-8153-3319-6
- Williams, Stephen; Friell, Gerard (1994). Theodosius: The Empire at Bay, B.T. Batsford Ltd. ISBN 0-300-06173-0