Diocletian, the Roman Emperor to end Rome’s third century crisis in imperial succession, is known best by Catholics for the launching last great persecution of Christians before Constantine’s so-called Edict of Milan in 313 AD. His administrative reforms, however, would have far reaching impact. For example, the tetrarchy (rule of four), which Diocletian instituted for smoother imperial succession, introduced a new east-west dichotomy which persists to this day.
Another good example is his introduction of the diocese.
The dioceses were established as part of Emperor Diocletian’s broader provincial reforms. In order to keep provincial governors from gaining too much power, which could once again plunge the Empire into another civil war, Diocletian doubled the number of Roman provinces from 50 to 100. But in order administer all 100 provinces efficiently, Diocletian grouped the provinces into twelve dioceses (see map). Unbeknownst to Diocletian, the term diocese would one day be used to describe the territory of a local church under a bishop, who is himself a successor to one of the twelve Apostles.
As imperial power began to collapse in “the West” after 476, the Church began to step in to fill the vacuum of stability – and it was natural for her to continue using the term diocese, even if the seven westernmost dioceses established by Diocletian would be broken up into smaller and smaller units. Nevertheless, with a bishop at the head of each diocese, the Church in the middle ages began to see herself as the rightful leader of both ecclesial and secular affairs rather than an interim caretaker during years of political and economic instability.
Perhaps it was providential that the anti-Christian Diocletian would establish the first twelve dioceses, but we must also remember that it was providential that many God-fearing men would establish the nations.