When did it become the official religion of the Roman Empire?

Answer 1

Mainstream Christianity was made the sole legitimate religion of the empire by the Edict of Thessalonica by co emperors Gratian, Theodosius I and Valentinian II in 380.

The Edict endorsed the creed Catholic Church, which at that time was a church which subscribed to the Nicene Creed, a particular interpretation of the trinity, and was opposed to rival interpretations of the trinity and other doctrines of Christianity. At that time it had two branches, Western or Latin, (which was the main religion in the western part of the empire) and Eastern or Greek (which was the main religion of the eastern part of the empire). Later these two churches spit (the Great Schism) and then they came to be called Catholic and Orthodox respectively.

The purpose of the edict was to ban dissident Christian doctrines, which were branded as heretic. Their persecution started soon after the issuing of the edict. The main target was Arian Christianity, which was popular around the empire.