Valentinian was succeeded by his son, Gratian, in 375. He was then only sixteen years of age. He admitted as a nominal colleague his half-brother, the younger Valentinian; and soon after he chose Theodosius as an active colleague, on whom he bestowed the sovereignty of the East. Gratian had been educated in the Christian faith, and gave evidence of being a true believer. He was the first of the Roman Emperors who refused the title and robe of high priest of the ancient religion. How could a Christian, he said, be the high priest of idolatry? It is an abomination to the Lord. Thus we see in the early piety of this young prince the blessed effects of the testimony of the faithful. What a new and strange thing in me; a pious prince to ascend the throne of Rothe Caesars at the age of sixteen! But he was humble as well as pious.
Being conscious of his own ignorance in divine things, he wrote to Ambrose, bishop of Milan, to visit him. "Come," he said, "that you may teach the doctrines of salvation to one who truly believes; not that we may study for contention, but that the revelation of God may dwell more intimately in my heart." Ambrose answered him in an ecstasy of satisfaction: "Most christian prince," he says, "modesty, not want of affection, has hitherto prevented me from waiting upon you. If, however, I was not with you personally, I have been present with my prayers, in which consists still more the duty of a pastor."
The young Emperor was generally popular; but his attachment to the orthodox clergy, the time he spent in their company, the influence they gained over him (especially Ambrose) exposed him to the contempt of the more warlike part of his subjects. The frontiers were sorely pressed at this time by the barbarians, but Gratian was unable to undertake the conduct of a war against them. Maximus, taking advantage of the disaffection of the army, raised the standard of revolt. Gratian, seeing the turn things had taken, fled, with about three hundred horse, but was overpowered and killed at Lyons in the year 383. Maximus, the usurper and assassin, placed himself on the throne of the West. He was afterward overthrown and slain by Theodosius, and the younger Valentinian placed upon the throne of his father.