Arians were a faction within the fourth-century church that denied the divinity of Christ. They vied with orthodox Christians for control of the Roman Empire. Constantine called the first ecumenical church council at Nicea to deal with the problem, but decades later Arians were more influential than ever. That changed when Theodosius came to imperial power. Reared in the orthodox tradition of the Nicene council, he determined to suppress Arianism. On this day February 27, 380, he issued an edict commanding all Christians to be catholic Christians—that is, to hold that the Father, Son, and Holy Spirit are one Godhead and equal in majesty. Theodosius summoned a council at Constantinople (the Second Ecumenical Council) to reaffirm the Nicene Creed and clarify the divinity of the Holy Spirit. He replaced Arian bishops throughout the East with orthodox bishops and expelled Arians from Constantinople. Jews and Jewish synagogues remained protected.
“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 [Bishop of Rome] 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 that in accordance with the will of Heaven we shall decide to inflict.”
“The Edict of Thessalonica.” Nov. 21, 2014 <http://www.sevencouncils.com/1/post/2013/02/the-edict-of-thessalonica.html>