That Constantine divided the Empire between his Three Sons, whom he had instructed in Politics and Religion
Having thus established his power in the opposite extremities of the world, he divided the whole extent of his dominions, as though he were allotting a patrimonial inheritance to the dearest objects of his regard, among his three sons. To the eldest he assigned his grandfather's portion; to the second, the empire of the East; to the third, the countries which lie between these two divisions. And being desirous of furnishing his children with an inheritance truly valuable and salutary to their souls, he had been careful to imbue them with true religious principles, being himself their guide to the knowledge of sacred things, and also appointing men of approved piety to be their instructors.
At the same time he assigned them the most accomplished teachers of secular learning, by some of whom they were taught the arts of war, while they were trained by others in political, and by others again in legal science. To each moreover was granted a truly royal retinue, consisting of infantry, spearmen, and body guards, with every other kind of military force; commanded respectively by captains, tribunes, and generals of whose warlike skill and devotion to his sons the emperor had had previous experience.
Source: Life of Constantine (New Advent)