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Dioceses of the Roman Empire, 400 AD
In the later organization of the Roman Empire, the increasingly subdivided provinces were administratively associated in a larger unit, the diocese (Latin dioecesis, from the Greek term διοίκησις, meaning "administration").
After the Christianity was given legal status in 313 the Churches began to organize themselves into dioceses based on provincial, not on the larger regional imperial districts. The dioceses were often smaller than the provinces since there were so many more bishops than governors. Christianity was declared Empire's official religion by Theodosius I 380. Constantine I in 318 gave litigants the right to have court cases transferred from the civil courts to the bishops. This situation must have hardly survived Julian, 361-363. Episcopal courts are not heard of again in the East until 398 and in the West in 408. The quality of these courts were low, and not above suspicion as the bishop of Alexandria Troas found out that clergy were making a corrupt profit. Nonetheless these courts were popular as people could get quick justice without being charged fees, A. H. M. Jones, Later Roman Empire, 1964, p. 480-481 ISBN 0-8018-3285-3. Bishops had no part in the civil administration until the town councils, in decline, lost much authority to a group of 'notables' made up of the richest councilors, powerful and rich persons legally exempted from serving on the councils, retired military, and bishops post-450 A.D. As the Western Empire collapsed in the 5th century|5th century]] bishops in Western Europe assumed a larger part of the role of the former Roman governors. A similar, though less pronounced, development occurred in the East, where the Roman administrative apparatus was largely retained by the Byzantine Empire. In modern times, many dioceses, though later subdivided, have preserved the boundaries of a long-vanished Roman administrative division. For Gaul, Bruce Eagles has observed that "it has long been an academic commonplace in France that the medieval dioceses, and their constituent pagi, were the direct territorial successors of the Roman civitates.
Modern usage of 'diocese' tends to refer to the sphere of a bishop's jurisdiction. This became commonplace during the self-conscious "classicizing" structural evolution of the Carolingian Empire in the 9th century, but this usage had itself been evolving from the much earlier parochia ("parish"), dating from the increasingly formalised Christian authority structure in the 4th century.