The Roman empire embraces Christianity

The Edict of Thessalonica ushers in a new state religion – and denounces heretics

Towardsthe end of the fourth century, the Roman empire was a place of tremendous religious ferment. After a long expansion in the empire’s cities, Christianity had received an enormous boost from Constantine the Great, who legalised it in AD 313 and raised it to the first rank of imperial religions. But Christianity itself was seized with bitter dissension, with rival Nicene and Arian bishops arguing about whether Jesus was of the same substance as God, had been created by him, or was merely a man.

So on 27 February 380, the emperors Gratian, Valentinian II and Theodosius I, joint rulers of the vastly overstretched Roman world, decided to impose some clarity. “It is our desire,” began their Edict of Thessalonica, “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.”

In other words, Christianity was now the official state religion of the Roman empire. But which kind of Christianity? “Let us believe,” the edict went on, “in the one deity of the Father, the Son and the Holy Spirit, in equal majesty and in a holy trinity. We authorise 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.”

But that, of course, did not stop the arguing.

A late fourth-century Roman mosaic showing Jesus with the apostles. After years of growth across the empire, Christianity was elevated to the first rank of imperial religions in 380