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.