Constantine the Great might have authorised Christianity across the Roman Empire with the Edict of Milan in 313 AD, but it was the emperor Theodosius I, half a century later, who put the brute force of the imperial state behind the faith.
Policy had vacillated through the 4th century. The emperor Julian (361-363) had been a pagan who disdained Christianity as a corruption of Judaism. Julian’s successor, Jovian (363-364), restored Christianity and made toleration imperial policy – including paganism, but not the performance of magic rites.
But what did it mean to be Christian? Most believers followed the rulings of the Council of Nicaea in 325 and accepted the Holy Trinity; the followers of Arius, a Libyan theologian, did not. And there was no guarantee that the right kind of Christian would occupy the imperial throne: the emperor Valens, an Arian, had reigned for 14 years, dying in battle in the summer of 378. Ironically he died at the hands of perhaps the most powerful group of Arian believers in history, the Goths.
So it was that on 27 February 380, the 23 year old Theodosius, born into the faith but only recently baptised after a serious illness, issued the Edict of Thessalonica, abjuring as heresy all teachings that contradicted the Nicene creed – and threatening not just divine punishment but the retributive power of imperial justice for those who strayed.
Five years later Priscillian, bishop of Avila, and his followers would be the first Christians to be executed by other Christians for their beliefs through the institutions of government. “What is truth?” Pilate had asked. After Theodosius, it was a matter of state.
This piece first appeared in the February 2021 Months Past column in History Today.