Known as The Great, (347-395 AD) Theodosius I was the son of the Spanish general Flavius Theodosius. He was working as a military governor in Moesia when his father was executed by Valentinian in 376. After his father's death, Theodosius withdrew from military life until 379, after which emperor Valens is defeated and killed by the Visigoths at Adrianople (todays Edirne) in 378 and emperor Gratian named him Augustus (co-ruler) of the East. In 381, he signed for an alliance with the Visigoths to keep them under control, who had been invading areas of the Byzantine Empire since 375 instead of becoming part of the Roman Army.
In 380, Theodosius issued a law ordering all citizens to believe in the Nicene Creed of the Council in 325 to be considered Catholic Christians. He severely acted against paganism like closing and even destroying temples and banning the Olympic Games tradition. In 381 he convened the second Ecumenical Council in Istanbul (First Council of Constantinople). In 390 AD Theodosius brought the Egyptian Obelisk carved from granite from the Temple of Luxor in Karnak to Istanbul and had it erected in the Hippodrome. In 391 and 392, Theodosius prohibited sacrifices and worshiping of pagan gods, making Christianity the only religion of the state. In 394, he fought his pagan enemies and won. Theodosius became ill and died in 395 AD. He divided the Roman Empire between his two sons.