One aspect of Christianisation was the Christianisation of sites that had been pagan. The landscape itself was Christianized, as prominent features were rededicated to Christian saints, sometimes quite directly, as when the island of Oglasa in the Tyrrhenian Sea was christened Montecristo.
In the first centuries of Christianity churches were either house churches in whatever houses were offered for use by their owners, or were shrines on the burial-sites of martyrs or saints, which following the usual classical practice were invariably on the (then) edges of cities - the necropolis was always outside the polis. In Rome the early basilica churches of St. Peter's, Saint Paul Outside the Walls and San Lorenzo fuori le Mura, all follow this pattern. This distinction was gradually broken down, perhaps earliest in Roman Africa, as relics of the saints came to be kept in city-centre churches. By the 6th century bishops were often buried inside their cathedral, and other Christians followed. After the Peace of the Church, the old pagan temples continued to function but gradually fell into disuse, and were finally all closed by the decrees of Theodosius I at the end of the 4th century. Initially they were shunned by Christians, perhaps because of their pagan associations, but also because their shape did not suit Christian requirements: "To the early Church, only one sort of building seemed suitable for christianization: the basilica", which had previously always been a secular type of building. Some of these basilicas were private ones in the homes of wealthy Christians: examples include the 4th century foundations of San Lorenzo in Damaso and the Basilica di San Clemente. Eventually the prime sites of the pagan temples were very often occupied for churches, the church of Santa Maria sopra Minerva (literally Saint Mary above Minerva) in Rome, Christianized about 750, being simply the most obvious example. However this process did not really begin in Rome itself until the 6th and 7th centuries, and was still under way during the Renaissance, when the Pantheon was made a church and Santa Maria degli Angeli e dei Martiri and San Bernardo alle Terme made from parts of the enormous Baths of Diocletian.
By the 7th century attitudes had changed and missionaries to the barbarian nations enthusiastically turned pagan sites immediately over to church use. Sulpicius Severus, in his Vita of Martin of Tours, a dedicated destroyer of temples and sacred trees, remarks "wherever he destroyed heathen temples, there he used immediately to build either churches or monasteries" (Vita, ch xiii), and when Benedict took possession of the site at Monte Cassino, he began by smashing the sculpture of Apollo and the altar that crowned the height. Montmartre (originally Mons Martis, "Mount of Mars", later re-interpreted as Mons martyris, "Mountain of the martyr") was the site of one of the oldest surviving Christian churches in France - Saint Pierre, and where the Jesuit movement was supposedly founded, was earlier a mercurii monte - a high place dedicated to Lugus, a major Celtic deity (and one that the Romans viewed as a homology of Mercury).
In Francia, the site chosen for the abbey of Luxueil were the ruins of a well-fortified Gallo-Roman settlement, Luxovium, that had been ravaged by Attila in 451, and was now buried in the dense overgrown woodland that had filled the abandoned site over more than a century; the place still had the advantage of the thermal baths ("constructed with unusual skill", according to Columbanus' early biographer, Jonas of Bobbio) down in the valley, which still give the town its name of Luxeuil-les-Bains. Jonas described it further: "There stone images crowded the nearby woods, which were honoured in the miserable cult and profane former rites in the time of the pagans". With a grant from an officer of the palace at Childebert's court, an abbey church was built with a sense of triumph within the heathen site and its "spectral haunts".
In country places and among country people (pagani) as Jean Seznec observes euhemerist dismissal by Christian writers of pagan deities as once having been human was insufficient cause to abandon old ways: "in country districts, the chief obstacle to Christianity was offered by the tenacious survival of anthropomorphic cults; here the problem became one of still further humanizing the divinities of springs, trees and mountains, in order to rob them of their prestige". In Britain and the Celtic northwest of Europe, the divinities of springs were transformed into local saints who were often venerated only at the location of their "holy well".
The conversion of pre-Christian places of worship, rather than their destruction, was particularly true of temples of Mithras, a religion that had been the main rival to Christianity during the 2nd and 3rd centuries, especially among the Roman legions. In Rome the early titular churches, each protected by a patron, were sometimes adapted from the basilica, or auidience hall, of a prominent man's domus.
During the Reconquista and the Crusades, the cross served the symbolic function of physical possession that a flag would occupy today. At the siege of Lisbon in 1147, when a mixed group of Christians took the city, "What great joy and what a great abundance there was of pious tears when, to the praise and honor of God and of the most Holy Virgin Mary the saving cross was placed atop the highest tower to be seen by all as a symbol of the city's subjection."