By the Edict of Thessalonica Three Roman Emperors Make Nicene Christianity the Official State Religion of the Roman Empire

On February 27, 380, by the Edict of Thessalonica, also known as Cunctos populos, Roman Emperors Theodosius I, Gratian, and Valentinian II made Nicene Christianity the official state religion of the Roman Empire, stating that all their subjects should profess the faith of the bishops of Rome and Alexandria.

The edict was issued shortly after Theodosius had suffered a severe illness in Thessalonica (Thessaloniki), and was baptized by Acholius, the bishop of that city.