In a few privileged cities, however, the impact of Christian building was more sudden and impressive. Imperial patronage, in particular, could bring about substantial and swift changes to a city’s religious topography. In Rome, after he captured the city in 312, Constantine set about honouring his God and the great martyrs with whom the capital was associated. He built two vast churches, each five aisles wide and some hundred metres long, one at the Lateran, and one on the Vatican hill over the grave of St Peter; and he and his family built further large churches at S. Croce, S. Agnese, S. Lorenzo and SS. Marcellino e Pietro. With Constantine, Rome retained its old secular and pagan monuments, but it also became a great Christian city, with a distinctive and obvious Christian monumentality. Before Constantine’s building-efforts, Christians had met in houses indistinguishable from other domestic buildings; thereafter they met in huge and elaborately decorated churches.(58)
The remarkable impact that the emperor’s Christian patronage had on Rome is best seen in the case of St Peter’s. Here Constantine wished to place the high altar directly over the place sanctified by tradition as the grave of the apostle. Before his work, this spot was marked only by a small aedicula in the middle of a busy necropolis of substantial family tombs. It must have been entirely unknown to all but the devout. As a result of Constantine’s work, the foot-slopes of the Vatican hill were levelled to form a great raised plat or platform some hundred metres long (or some two hundred metres long, if as seems probable, the atrium was his); the necropolis was closed down and covered over; and a great five-aisled basilica was constructed, with its high altar directly over St Peter’s resting-place.(59) This massive feat of civil engineering and construction was carried out in clear view of the centre of the city across the river, and in the immediate vicinity of a favoured shrine of the senatorial aristocracy, where they went to perform the rituals of the Magna Mater.(60) Before Constantine, most Romans had probably never heard of St Peter and would certainly have disapproved of venerating any mortal remains, let alone the mortal remains, of an executed criminal; after Constantine, like it or not, Peter and his bones assumed the integral and major role in the city’s life that they have retained ever since.
However, even in as dramatic a case as Rome, with a very rich bishopric and access to imperial power and imperial funds, it took centuries for the new religion to transform the whole topographical shape of them city. Constantine and his successors did not build churches in the heart of Rome, perhaps in part out of deference to its largely pagan aristocracy, but also probably through an uncertainty as to how to approach religious building within the centre of a city whose monuments were often explicitly pagan and yet held powerful and important historical resonance. Indeed in the earlier fourth century, building at the new Christian shrines was still accompanied by more traditional building on a substantial scale in the city’s centre. Constantine himself built large new baths on the Esquiline; and the senate built for him the famous arch which still stands by the Colosseum, and dedicated to him two massive projects undertaken by Maxentius – the vast basilica to the north-east of the Forum, and, more surprisingly, the huge rebuilt temple of Venus and Rome, the largest in the city.(61)
Only at the very end of the fourth century had the emphasis in new building moved definitely to the Christian shrines. In the years around 400, the emperors provided St Paul with a church on the Via Ostiensis to match that of his colleague Peter; and, most strikingly, the two triumphal arches built in this period (in 379/83 and 405) abandoned the traditional triumphal route that culminated in the Capitol for a new ‘Via Sacra’, the approach-road to St Peter’s.(62) But, although the emphasis of new building moved to the Christian shrines, the centre of the city remained dominated by the vast buildings, secular and pagan, of earlier centuries. Here, in the centre, only very slowly through the fifth and succeeding centuries were the traditional monuments abandoned and adapted, and only very slowly were great Christian basilicas built (such as S. Maria Maggiore in 432/40).