Theodosius I

On 27 February 380 an edict was issued at Thessalonica in the names of three emperors and addressed to the people of Constantinople. It opens with the words by which it is known, cunctos populos, all peoples, and it orders that all the peoples under the rule of the emperors' clemency shall hold the religion which Peter delivered to the Romans and which is now professed by Damasus of Rome and Peter of Alexandria: one deity of Father, Son and Holy Spirit in equal majesty and holy Trinity. Those who follow this are to be called catholic Christians. The rest are declared demented and insane, to bear the infamy of heretical teaching, and to expect punishment first from divine vengeance and then from the imperial authority.

Two things about it are worth stating precisely. The first is what it changed. Constantine had made Christianity legal and favoured; this made one version of it the religion of the state, defined it by naming two living bishops rather than by quoting a creed, and put the empire's coercive apparatus behind the definition. The second is what it did not do on its own. Enforcement took decades, and the immediate target was the Arian and Homoian Christianity that had held imperial favour for most of the preceding half century, rather than paganism.

The council Theodosius convoked at Constantinople the following year completed the work, revising the Nicene creed into the form recited in most churches today and settling the status of the Holy Spirit, which Nicaea had left almost unaddressed.

In 390 a Gothic garrison commander was lynched by a crowd in Thessalonica after arresting a popular charioteer. The reprisal was carried out in the hippodrome: the population was invited to the games and then attacked by soldiers, and the sources report thousands dead. The order came from the emperor, and the accounts say he tried to countermand it too late.

Ambrose, bishop of Milan, where the court then was, withdrew from the emperor's presence and wrote him a private letter saying that he could not offer the eucharist in his presence until he had done penance, and that he wrote as one who did not wish to shame him publicly. Theodosius submitted. He laid aside the imperial insignia, did public penance in the church at Milan over a period of months, and was restored to communion.

The episode became one of the most cited events in the whole medieval argument about the two powers, and it is worth handling carefully, because the version everyone knows is not the earliest. Ambrose's own letter survives and is restrained. The dramatic scene of a bishop barring an emperor at the church door and confronting him in front of a crowd comes from Theodoret, writing decades later, and reads as an improvement. What is not in doubt is the substance: a bishop imposed a penance on a reigning emperor for an act of state, and the emperor accepted it, which had not happened before.