After a prosperous reign of twenty years, becoming weary of the cares of state, Diocletian abdicated the throne, and forced or induced his colleague Maximian also to lay down his authority on the same day. Galerius and Constantius were, by this act, advanced to the purple and made Augusti; and two new associates were appointed as Cæsars. Diocletian, having enjoyed the extreme satisfaction of seeing the imperial authority quietly and successfully transmitted by his system, without the dictation of the insolent prætorians or the interference of the turbulent legionaries, now retired to his country-seat at Salona, on the eastern shore of the Adriatic, and there devoted himself to rural pursuits. It is related that, when Maximian wrote him urging him to endeavor, with him, to regain the power they had laid aside, he replied: "Were you but to come to Salona and see the vegetables which I raise in my garden with my own hands, you would no longer talk to me of empire." REIGN OF CONSTANTINE THE GREAT (A.D. 306-337); THE EMPIRE BECOMES CHRISTIAN.--Galerius and Constantius had reigned together only one year, when the latter died at York, in Britain; and his soldiers, disregarding the rule of succession as determined by the system of Diocletian, proclaimed his son Constantine emperor. Six competitors for the throne arose in different quarters. For eighteen years Constantine fought to gain supremacy. At the end of that time every rival was crushed, and he was the sole ruler of the Roman world. Constantine was the first Christian emperor. He was converted to the new religion--such is the story--by seeing in the heavens, during one of his campaigns against his rivals, a luminous cross with this inscription: "With this sign you will conquer." He made the cross the royal standard; and the Roman legions now for the first time marched beneath the emblem of Christianity. By a decree issued from Milan A.D. 313, Christianity was made in effect the state religion; but all other forms of worship were tolerated. With the view of harmonizing the different sects that had sprung up among the Christians, and to settle the controversy between the Arians and the Athanasians respecting the nature of Christ,--the former denied his equality with God the Father,--Constantine called the first OEcumenical, or General Council of the Church, at Nicæa, a town of Asia Minor, A.D. 325. Arianism was denounced, and a formula of Christian faith adopted, which is known as the Nicene Creed.