The Edict of Thessalonica, (also known as the De fide Catholica) is an Edict that effectively made Nicene Christianity, the State Religion and Church of the Roman Empire. It was issued by Roman Emperors Theodosius I, Gratian and Valentinian II in AD 380. The Edict was intended to oppose the Arian heresy. The Edict of Thessalonica in AD 380 would be followed by the First Council of Constantinopolis convoked by Theodosius I and attended by Bishops in AD 381.
IMPERATORES GRATIANUS, VALENTINIANUS AND THEODOSIUS AUGUSTI. EDICT TO THE PEOPLE OF CONSTANTINOPOLIS.
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 GRATIANUS AUGUSTUS AND FIRST OF THEODOSIUS AUGUSTUS.
- Codex Theodosianus XVI.1.2