The Edict of Thessalonica (also known as Cunctos populos), issued on 27 February AD 380 by Theodosius I, made the Catholicism of Nicene Christians the state church of the Roman Empire. It condemned other Christian creeds such as Arianism as heresies of "foolish madmen," and authorized their punishment.
Slight correction regarding video at 7:50 :- the council at AD381 was the Council of Constantinople
Source of video: thehumanjesus