From The Inheritance of Rome by Chris Wickam, page 55:
But in this case change was possible, all the same. For a start, whereas to pagan eyes an entire landscape could be numinous, to Christian eyes only specific cult-sites were so, points of light in an otherwise secular space. These were always, or soon became, churches, so they were highly visible. Few churches were ever built directly on or in temples, and those few were almost all urban. In cities, indeed, Christian topographies were in general rather more different from those of the pagans. Traditional public religion had been focused on the ceremonial buildings around the forum in the centre of the city, but churches for Christian worship were often on the edges of town, or outside, in cemetery areas. Urban religious activity became much more decentralized as a result, and cities even became spatially fragmented in some parts of the empire (in Gaul in particular), with little settlement nuclei around scattered churches, and in some cases a traditional city centre left in ruins. This was sometimes because city centres seemed just too pagan, or too secular; in Rome, major Christian capital though it became, no church was built in the wide forum area until 526. It was also linked to some real changes in ideas of the sacred, and of what caused spiritual pollution. Traditional Graeco-Roman religion regarded dead people as very dangerous and polluting; no adult could be buried inside city walls or in inhabited areas, and cemeteries were all beyond the edges of settlements. Martyrs and other saints were seen by Christians as different, however: not as sources of pollution, but the opposite, as people to venerate (in some cases, indeed, as not really dead).
[This item is cross-posted at Junior Ganymede.]