307-337 AD Ancient Rome, Constantine I The Great Ancient Grades AU. Constantine I, also known as Constantine the Great, was Roman emperor from AD 306 to 337. He was the first emperor to convert to Christianity. Born in Naissus, Dacia Mediterranea (now Niš, Serbia), he was the son of Flavius Constantius, a Roman army officer of Illyrian origin who had been one of the four rulers of the Tetrarchy. The age of Constantine marked a distinct epoch in the history of the Roman Empire and a pivotal moment in the transition from classical antiquity to the Middle Ages.