A Roman or civil diocese (Latin: dĭœcēsĭs, from the Greek: διοίκησις, "administration") was one of the administrative divisions of the later Roman Empire, starting with the Tetrarchy. It formed the intermediate level of government, grouping several provinces and being in turn subordinated to a praetorian prefecture.
The earliest use of 'diocese' as an administrative unit was in the Greek-speaking East. Three districts— Cibyra, Apamea and Synnada— were added to the province of Cilicia in the time of Cicero, who mentions the fact in his familiar letters (EB 1911). The word 'diocese', which at that time was equivalent to a tax-collecting district, came to be applied to the territory itself.
The reorganization of the Roman Empire known as Tetrarchy was initiated by Emperor Diocletian in the 290s. He divided the existing provinces into smaller, more compact and easily controllable units, with a greatly increased bureaucracy. The provinces were in turn grouped into twelve dioceses, each headed by a vicarius [dioeceseos], i.e. a vicar. These diocese themselves were grouped into four huge prefectures, each one under a powerful civil official called a praetorian prefect. Under the tetrarchic system, each of the two senior emperors (Augusti) had a praetorian prefect. The largest diocese, Oriens, included sixteen provinces, and the smallest, Britannia, comprised only four provinces.
After the establishment of definite praetorian prefectures in the mid-4th century, the dioceses functioned as the intermediate level between the province and the prefecture, although the hierarchy was not rigid: provincial governors could appeal directly to the praetorian prefect or the emperor, and vice versa. In the West, the dioceses were disbanded as Roman power receded, but in the East, they survived. Seeing their role as somewhat ineffectual, the emperor Justinian I abolished most of the dioceses in his great reform in the 530s, preferring to strengthen the authority of provincial governors. This practice was extended to the recovered territories of Italy and Africa, where Justinian preferred to install praetorian prefects directly overseeing the respective provinces.
Introduction of the term in ecclesiastical usage
Between the 4th and 6th centuries, as the older administrative structure began to crumble, the role of the bishops in the western lands of the Empire enabled those lands and their peoples to maintain a semblance of civilisation as the authority of Rome vanished. The senatorial aristocracy, especially in the provinces, continued in many places to serve as sources of local authority to complement the authority assumed by the Church. In Late Antiquity, political power often came to be vested in the spiritual offices of the bishops in each region. This transfer of authority from secular officials to ecclesiastical leaders was natural in that, because of the close integration of the secular and ecclesiastical leadership in the Empire, the areas of ecclesiastical administration always coincided with those of the Roman civil administration.
It is, therefore, unsurprising that, as the Catholic and Eastern Orthodox churches began to define their administrative structures, they relied on the older Roman terminology and methods to describe administrative units and hierarchy, which often caused the division between ecclesiastical and secular authority to disappear. In the Eastern Empire, this became fundamental doctrine: see Caesaropapism and State church of the Roman Empire.
A millennium later this process would be somewhat repeated when the Ottoman Empire conquered the Eastern Roman Empire (see Christianity and Judaism in the Ottoman Empire) and the eastern bishops assumed political roles as the Roman civil structure was stripped away. In modern times, many an ancient diocese, though later divided among several dioceses, has preserved the boundaries of a long-vanished Roman administrative division.
- Diocese, the ecclesiastical territory originally corresponding to a civil diocese