Diocese (Gr. administration), in ancient times, an administrative division of the Roman empire, forming a subdivision of one of the four prefectures, and comprising several provinces; in modern language, the territory governed by a bishop. As early as the time of Cicero we find mention of the dioceses, or districts, of Asia Minor. Subsequent to the reorganizations of the empire under Diocletian and Constantine the Great, the dioceses were the East, Egypt, Asia, Pontus, Thrace, Macedonia, Dacia, Illyria, Italy, Africa, Gaul, Spain, and Britain. The East was governed by a count, Egypt by a prefect, some by proconsuls, and others by vicars. Each province was subdivided into cities (civitates), subject to a supreme magistrate residing in the chief city or metropolis. When the gospel began to be preached, each city among the Greeks and Latins was governed by magistrates chosen from among the citizens, and sometimes designated as senate, sometimes as ordo or curia. Over this governing body presided a superior magistrate called dictator or defensor of the city. His authority, and that of his brother magistrates, extended over the adjacent territory, made up generally of a number of towns and villages. The first administration of the church was moulded on this civil division. In each civitas, or city, with its suburban territory, there was established a corresponding ecclesiastical magistracy, namely, a presiding officer (episcopus, bishop), with a senate of priests (presbyterium); and his spiritual jurisdiction extended as far as the civil jurisdiction of the city, its circle being called at first parish, but from the beginning of the 4th century diocese. As each city of the empire had in the towns (oppida) of its jurisdiction magistrates subordinate to those of the city itself, so the church in each city came to have ministers subordinate to the bishop in these towns ; hence the origin of parishes and parish priests. In like manner the capital city of each province came to have its ecclesiastical metropolitan, who had jurisdiction and superintendence in things spiritual over the bishops of the province. And in some respects corresponding with the office of vicar or proefectus proetorio in each civil diocese of the empire, there arose the dignity of patriarch or exarch in the church, whose preeminence extended over a number of ecclesiastical provinces. At present, in the Roman Catholic and Greek churches, the word diocese means "the territory attached to each see," whether patriarchal, primatial, metropolitan, or episcopal. Thus, the pope is bishop of the diocese of Rome; the patriarch of Lisbon, of the city of that name and its ecclesiastical territory, forming the diocese of Lisbon; the archbishop of New York, of the city of New York and that portion of the state forming together the archdiocese.
In the Protestant Episcopal churches, a diocese is the dis-. trict ruled by a bishop. In the Evangelical church of Germany, a diocese is a combination of parishes under the care of a superintendent. DIOCLETIAN (Diocletianus Valerius), a Roman emperor, born near Salona in Dalma-tia, A. D. 245, died in that town in 313. From his mother, who was called Doclea or Dioclea, from the village in which she lived, he derived the name Docles or Diocles, which he changed on assuming imperial authority to Diocletia-nus, taking at the same time the patrician name of Valerius. His parents were of the humblest class; but his abilities secured his rapid promotion in the army, which he entered at an early age, and his personal popularity with the troops gave him the greatest influence. He held important positions under Probus and Aurelian, and served under Carus in the expedition against Persia, which was suddenly terminated by that emperor's death in his camp on the banks of the Tigris, in 284. When, during the retreat that followed, Numerian, the son of Carus, was assassinated, the soldiers unanimously chose Diocletian as his successor.
But he was obliged to contest his position with Carinus, brother of Numerian, who was recognized as emperor in Europe. The armies of the hostile sovereigns met near Margum, not far from the Danube in Moesia, where the battle itself was decided against Diocletian; but Carinus, eagerly following the flying enemy, was killed by one of his own officers, and his array readily acknowledged Diocletian as his successor. He was installed as emperor with great ceremony at Nicomedia. But affairs were still in the greatest confusion, and he determined to associate with himself a colleague in the supreme dominion, and fixed his choice on Maximian, his old companion in arms, a rough barbarian, whom he invested with the imperial dignity in 280, and in whom he found a useful assistant and a constant friend. The Roman empire was beset with enemies and torn by factions. The peasants of Gaul rose in arms; Mauritania was in rebellion; Egypt was disturbed by external enemies and internal convulsions; while all along the frontier, from the Euphrates to the Rhine, the barbarians were threatening to destroy the empire by their invasions.
Maximian subdued the Bagaudae or Gallic peasants, but Diocletian determined to strengthen the empire by raising two more Roman soldiers to the purple, Galerius, son of a Dacian shepherd, and Constantius, surnamed Chlorus, son of a noble Moesian, and father of Constantine the Great. These two princes in 292 received the title of Caesar, and having repudiated their wives, Galerius married the daughter of Diocletian, and Constantius the stepdaughter of Maximian. Britain, Gaul, and Spain were assigned to Constantius; Galerius received the Illyrian and Danubian provinces; Italy and Africa, with Sicily and the islands of the Tyrrhenian sea, were held by Maximian; while Diocletian, the head of all, retained under his own dominion Thrace, Egypt, and the provinces of Asia, and established his capital at Nicomedia. By this arrangement, on the death of either of the Augusti, as Maximian and Diocletian were styled, the Caesar who had been associated with him was to be his successor, and another Ca3sar was to be appointed. These four princes, it was thought, would hold one another in check, so that no one of them would be able to attain to uncontrolled power. The plan was for a time successful.
Maximian subdued the rebellious provinces of western Africa; Diocletian reduced and secured Egypt; Galerius not only, under the superintendence of his father-in-law, compelled the Persians to make a treaty which secured the frontiers of that part of the empire for 40 years, but also vigilantly guarded the Danubian frontier; while Constantius invaded Britain, which for several years had been detached from the rest of the empire under the rule of Carausius, and restored that island to the control of the Roman emperors. After a prosperous reign of about 21 years, Diocletian, moved by his infirm health, or, as some writers have said, by the persuasions or menaces of his son-in-law Galerius, voluntarily resigned the throne in 305, and retired to Salona in his native country, where ho passed the remaining eight years of his life in retirement. Maximian, according to a previous agreement, abdicated at the same time, but was not contented in a private station, and a few years later wrote to his former colleague, proposing to him to resume the reins of government.
The reply of Diocletian has become celebrated. "Would you could see," he said, "the cabbages planted by my hand at Salona; you would then never think of urging such an attempt." - Diocletian struck a severe blow at the weaning influence of the senate by the removal of his court from Rome to Nicomedia, reduced the numbers and the importance of the praetorian guards, divided the provinces so as to lessen the power of the provincial governors, and increased the dignity and ceremony with which the emperor was surrounded. He is censured for permitting the persecution of the Christians; but it must be remembered that the greater part of these persecutions took place after Diocletian had resigned his authority. The history of the reign of Diocletian is exceedingly confused, and only the principal events given above can be assumed to be accurate. Authorities differ widely in their account of many of the details. The year 284, the period of Diocletian's accession, was made by the ecclesiastical writers the beginning of an era called " the era of Diocletian;" a chronological form often employed in early theological works.