The edict of Thessalonica in 380 made Nicene Christianity the religion of the Roman state, and it did so by naming the party it meant: those who follow the faith of Peter as taught by Damasus of Rome and Peter of Alexandria are to be called catholic Christians, and the rest are heretics. It is the first time the word does legal work rather than theological work. Everything downstream — imperial councils, imperial exiles, a church with a magistrate behind it — begins in that sentence.
Codex Theodosianus (book XVI) · CTh 16.1.2