Constantine I is the only Roman emperor called “The Great”. Born around A.D. 272 at Naissus, in modern Serbia, his father was Constantius I, one of the soldier emperors in the Tetrarchic system. On campaign in Britain the sick Constantius I died at York in A.D. 306 so the army proclaimed Constantine emperor. From A.D. 307 Constantine styled himself as “Augustus” and slowly but surely became the sole ruler of the empire. Notably, at the Milvian Bridge in A.D. 312, he defeated Maxentius with a Christian symbol on his soldier’s shields, apparently given by God in a dream.
As he aged Constantine left the old gods behind, becoming Christian, and this support shaped our modern world as Christianity in the medieval era was the common cause that united the west. He was baptised just before his death in A.D. 337 at roughly 65.