Few cities can boast that they have served as the capital of an empire extending to three continents, and fewer still can claim that they have played an important role on the world stage for more than two millennia. In fact, there is probably only one: Rome, the ‘Eternal City’ not only in name. Rome’s unique position as a site of continuous and continuing significance is due in part to its geography, and in part to the vicissitudes of its history. But the city's longevity also owes much to its remarkable ability to ‘re-invent’ itself, as appropriate to changing circumstances. And the most significant moment of ‘re-invention’ is also perhaps one of the least studied: the period of the early Middle Ages. No longer the political capital of a far-flung empire, Rome re-invented itself as the urbs sacra, the focal point of a powerful world religion, acquiring a new lease on life that continues to this day.

This process of re-invention did not happen overnight; indeed, it would evolve over many centuries. In the fourth and early fifth centuries CE, as Richard Krautheimer has demonstrated, the physical imprint of Christianity was mostly to be found on the edges of the city, and outside the circuit of the Aurelian walls, focused on the Lateran basilica and the adjacent residence of the bishops of Rome, and the suburban cemeteries with their attached churches and monasteries. The primary patronage of new building activity remained initially lay, not clerical, although gradually extending outwards from the imperial family itself to embrace members of the aristocracy and others with wealth or authority.