For another saint called Theodosius the Great living in the fifth-sixth centuries, see Theodosius the Great (cenobiarch). The holy and right-believing Emperor Theodosius I, also called Theodosius the Great, was the last emperor who ruled over both the eastern and western portions of the Roman Empire. He ruled from 379 to 395. Theodosius I is also known for making Christianity the official state religion of the Roman Empire. He was a strong defender of the Orthodox Christian faith and is a saint. His feast day is celebrated on January 17.