To many of the Christians, on the other hand, long persecution had doubtless taught the wisdom of recanting the extremes of doctrine which had made even sceptical statesmen regard them as a danger to any State. It is clear that bishops like Eusebius of Cæsarea would readily promise to the government a loyal attention to its interests in the event of its tolerating and befriending the Church; and the sacred books offered texts for any line of public action. The empire, always menaced by barbarism on its frontiers, needed every force of union that could be used within; and here, finally adaptable to such use, was the one organization that acted or was fitted to act throughout the whole. To the leading churchmen, finally, association with the State was the more welcome because on the one hand general persecution would cease, and on the other all the party leaders could hope to be able by the State's means to put down their opponents. A generation before, in the year 272, the Emperor Aurelian, on the express appeal of the party of bishops who had deposed Paul of Samosata, had intervened in that quarrel to give effect to the will of the majority, which otherwise could not have been put in force; and such occasions were sure to arise frequently. It needed only another innovating emperor to bring about the coalition thus prepared.
On the abdication of the co-emperors Diocletian and Maximian, the Cæsars, Galerius and Constantius Chlorus, became the Augusti; the former, as senior, taking the East, and the latter the West. At once the plans of Diocletian began to miscarry; and Galerius, instead of raising to the Cæsarship, as the other had wished, Maxentius the son of Maximian and Constantine the already distinguished son of Constantius, gave the junior titles to his nephews Severus and Maximin. The speedy death of Constantius, however, secured the election of Constantine to the purple by his father's troops in Britain; and there ensued the manifold strifes which ended in Constantine's triumph. Maxentius, and his father, who returned to power, put down Severus; and Maximian gave his daughter as wife to Constantine, thus creating a state of things in which three emperors were leagued against a fourth and one Cæsar. Soon Maximian and Maxentius quarrelled, the father taking refuge first with Constantine and later with Galerius; who, however, proceeded to create yet another emperor, Licinius. Immediately the Cæsar Maximin revolted, and forced Galerius to make him Augustus also. The old Maximian in the meantime went to league himself afresh with Constantine, who, finding him treacherous, had him strangled. Soon after, Galerius dying (in 311), Maximin and Licinius joined forces; while Maxentius, who held Italy and Africa, professing to avenge his father, declared war on Constantine, who held Gaul. The result was the defeat and death of the former, leaving Constantine master of the whole West (312). In 314 he fell out with Licinius, who had in the meantime destroyed Maximin, and won from him Illyrium, Macedonia, and Greece. For ten years thereafter Constantine divided the empire with Licinius; then, quarrelling afresh with his rival, he captured and strangled him, and was sole autocrat (324).