Emperor Theodosius I of Constantinople/Istanbul (AD 347-395)
Theodosius I reigned as emperor for 16 years, from 379 to 395. He was a Nicene orthodox Christian and opposed the false teaching of Arianism. In 381, he convened Constantinople’s Ecumenical Church Council in the Church of Holy Peace (Hagia Irene), where the final version of the Nicene Creed was written.
In 381, Theodosius I established Christianity as the official religion of the Roman empire—not Constantine the Great, as many mistakenly think. Theodosius I also closed down many of Asia Minor’s pagan temples in 391. He was the last emperor to rule over an undivided Roman empire. After his death in January 395, his son Arcadius ruled over the eastern Roman empire, while his other son, Honorius, inherited the western Roman empire.