He was a Roman Emperor and persecutor of the Church, borne of parents who had been slaves, at Dioclea, near Salona, in Dalmatia, A.D. 245; died at Salona, A.D. 313. Our Coptic Orthodox Church began its own calendar of Martyrs by the first year of his reign 284 AD.
He entered the army and by his marked abilities attained the offices of Governor of Mœsia, consul, and commander of the guards of the palace. In the Persian war, under Carus, he especially distinguished himself. When the son and successor of Carus, Numerian, was murdered at Chalcedon, the choice of the army fell upon Diocletian, who immediately slew with his own hand the murderer Aper (17 Sept., 284). His career as emperor belongs to secular history. The reign of Diocletian (284- 305) marked an era both in the military and political history of the empire. The triumph which he celebrated together with his colleague Maximian (20 Nov., 303) was the last triumph which Rome ever beheld. Britain, the Rhine, the Danube, and the Nile furnished trophies; but the proudest boast of the conqueror was that Persia, the persistent enemy of Rome, had at last been subdued. Soon after his accession to power Diocletian realized that the empire was too unwieldy and too much exposed to attack to be safely ruled by a single head. Accordingly, he associated with himself Maximian, a bold but rude soldier, at first as Cæsar and afterwards as Augustus (286). Later on, he further distributed his power by granting the inferior title of Cæsar to two generals, Galerius and Constantius (292). He reserved for his own portion Thrace, Egypt, and Asia; Italy and Africa were Maximian's provinces, while Galerius was stationed on the Danube, and Constantius had charge of Gaul, Spain, and Britain. But the supreme control remained in Diocletian's hands. None of the rulers resided in Rome, and thus the way was prepared for the downfall of the imperial city. Moreover, Diocletian undermined the authority of the Senate, assumed the diadem, and introduced the servile ceremonial of the Persian court. After a prosperous reign of nearly twenty-one years, he abdicated the throne and retired to Salona, where he lived in magnificent seclusion until his death. Diocletian's name is associated with the last and most terrible of all the ten persecutions of the early Church. Nevertheless it is a fact that the Christians enjoyed peace and prosperity during the greater portion of his reign. Eusebius, who lived at this time, describes in glowing terms "the glory and the liberty with which the doctrine of piety was honored", and he extols the clemency of the emperors towards the Christian governors whom they appointed, and towards the Christian members of their households. He tells us that the rulers of the Church "were courted and honored with the greatest subserviency by all the rulers and governors". He speaks of the vast multitudes that flocked to the religion of Christ, and of the spacious and splendid churches erected in the place of the humbler buildings of earlier days. At the same time he bewails the falling from ancient fervor "by reason of excessive liberty". Had Diocletian remained sole emperor, he would probably have allowed this toleration to continue undisturbed. It was his subordinate Galerius who first induced him to turn persecutor. These two rulers of the East, at a council held at Nicomedia in 302, resolved to suppress Christianity throughout the empire. The cathedral of Nicomedia was demolished (24 Feb., 303). An edict was issued "to tear down the churches to the foundations and to destroy the Sacred Scriptures by fire; and commanding also that those who were in honorable stations should be degraded if they persevered in their adherence to Christianity". Three further edicts (303-304) marked successive stages in the severity of the persecution: the first ordering that the bishops, presbyters, and deacons should be imprisoned; the second that they should be tortured and compelled by every means to sacrifice; the third including the laity as well as the clergy. The atrocious cruelty with those edicts was enforced, and the vast numbers of those who suffered for the Faith are attested by Eusebius and the Acts of the Martyrs. We read even of the massacre of the whole population of a town because they declared themselves Christians. The abdication of Diocletian (1 May, 305) and the subsequent partition of the empire brought relief to many provinces. In the East, however, where Galerius and Maximian held sway, the persecution continued to rage. Thus it will be seen that the so-called Diocletian persecution should be attributed to the influence of Galerius; it continued for seven years after Diocletian's abdication.