Roman emperor Theodosius I made Nicene Christianity the empire’s official state religion through the Edict of Thessalonica in 380 CE.
The edict, issued with Gratian and Valentinian II, declared that Roman citizens should follow the faith associated with the bishops of Rome and Alexandria. It specifically endorsed the Nicene creed and rejected several competing Christian positions, including Arian theology.
This development came after Constantine I had legalized Christianity in the early fourth century. Constantine’s policy ended major imperial persecution, but it did not immediately make Christianity the empire’s official religion. Pagan practices and other Christian traditions continued for decades.
Theodosius later supported measures against public pagan sacrifices and temples, although religious change was gradual and uneven across the empire. The Edict of Thessalonica should therefore not be confused with the Edict of Milan of 313, which granted religious toleration rather than establishing Christianity as the state faith.