A Roman diocese was a group of provinces created in the late third century, probably by Diocletian. Each was run by an official called a vicar. Dioceses sat between the provinces and the praetorian prefectures. They are not drawn on our map.
Key takeaways
- Many scholars date the dioceses to about AD 296 to 297, though the date is still debated [1].
- There were first twelve dioceses, rising to fourteen by the end of the fourth century [1].
- Our map draws 43 province outlines from about AD 200, before dioceses existed, so dioceses are not on it.
- Each diocese was headed by a vicar who supervised provincial governors and heard appeals [1].
How dioceses worked
By the late third century, Roman provinces had been cut into smaller pieces. Provinces were multiplied from 48 to about 100 [1]. Small provinces were easier to control, but there were now too many for the emperors to supervise directly. The answer was a new layer above them: the diocese, a group of neighbouring provinces.
Many modern scholars date the creation of the dioceses to about AD 296 to 297, in the reign of Diocletian, although one scholar argues for about 313 to 314, and the question remains controversial [1]. The empire was first divided into twelve dioceses, rising to fourteen by the end of the fourth century. The largest, the Diocese of the East, covered sixteen provinces [1].
Each diocese was headed by a vicar (vicarius), who acted for the praetorian prefect. Vicars supervised the provincial governors and heard appeals from their courts. They had no military powers: soldiers answered to a separate commander [1]. Some places stayed outside the system, including the proconsular provinces of Africa and Asia and the cities of Rome and Constantinople [1].
So the chain of command in the later empire ran from the emperor to the praetorian prefect, to the vicar of a diocese, and down to the governor of a province [3]. Later in the fourth century the prefectures became regional units that each contained several dioceses, which made the dioceses less important [1].
What the map does and does not show
Our map does not draw dioceses. The province layer shows 43 outlines based on the Ancient World Mapping Center's map of the empire around AD 200, which is before these reforms. For later dates, such as AD 395, the map colours the East and West separately, but the lines inside them are the older provinces. Do not read those outlines as a map of the later system. See the borders section of the homepage for how the outlines were built.
An example from our data
Take Britannia. Our data gives its annexation as AD 43 and its loss as AD 410, and draws it as one outline covering 170,292 km². By the later empire the island had been divided into several provinces, grouped into a diocese of Britain. In the prefecture of Gaul, that diocese sat alongside the dioceses of Gaul, Viennensis and Hispania [3]. The map's single outline therefore stands for what was by then several provinces under one vicar.
Syria is the other way round. Its outline covers 87,517 km² with its capital at Antiochia, and in the later empire the region fell inside the huge Diocese of the East [1]. For the second half of the story see the entry on the Eastern Roman Empire and the page for AD 395.
The thing people get wrong
Because the word is now used by churches, people assume a Roman diocese was a religious area ruled by a bishop. It was a civil district run by an official. The Christian church later borrowed the idea and the name, and kept using it long after the empire in the West had gone.
A second mix-up is to see dioceses as something the map is missing by mistake. They did not exist at the dates of our early outlines, and they were administrative groupings of provinces rather than lands with borders of their own.
Frequently asked questions
Is a Roman diocese the same as a church diocese?
Not originally. The Roman diocese was a civil unit of government. The church later borrowed the word for the area under a bishop.
Are dioceses on the map?
No. The map draws the provinces of about AD 200. Dioceses, prefectures and the many smaller late provinces are not drawn.
Who ran a diocese?
A vicar, who answered to a praetorian prefect. Vicars had no soldiers of their own, because troops were under a separate commander.
Sources
- Roman diocese, Wikipedia.
- Roman province, Wikipedia.
- Praetorian prefecture, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against Ancient World Mapping Center outlines and Pleiades).