The Christians of Britain inherited an island rich in localized mythology, from the mists of prehistory represented by Stonehenge and other megaliths to the sacred rivers of the Celtic era. In Christian theology, though, there was no space for these space-specific cults; the goal of the Church was to have a universal faith, and God could be found equally in any space. How, then, did Christians bridge the gap between old and new practices? Did they denounce the old practices, transform them, or ignore them and set out to find completely new places of worship? Did they still conceive of Britain as a world full of natural sanctity, or did they have new ideas about the purpose of nature?
The first major writer of Christian Britain was Gildas, who included a brief history of the country in order to analyze it and denounce its current state. In a short passage at the very beginning of his jeremiad, he carefully steps around the issue of pre-Christian conceptions of religion, but nonetheless gives us some hints of their idea of what Mircea Eliade called "sacred space":
I shall not speak of the ancient errors, common to all races, that bound the whole of humanity fast before the coming of Christ in the flesh. I shall not enumerate the devilish monstrosities of my land, numerous almost as those that plagued Egypt, some of which we can see today, stark as ever, inside or outside deserted city walls: outlines still ugly, faces still grim. I shall not name the mountains and hills and rivers, once so pernicious, now useful for human needs, on which, in those days, a blind people heaped divine honors.
The last sentence of the short paragraph is by far the most interesting, because we can extract from Gildas' stubborn resistance some real familiarity with the practices of pre-Christian Britain. Based on what we know from Romano-British archaeology, there would have been plenty of non-Christian "devilish monstrosities" carved into centuries-old stone for Gildas to shudder over, but there is little to suggest that there was recorded evidence of sacred places. It is safe to say that he could not have determined from the evidence of stonework alone that the Britons worshiped nature before the arrival of Christianity; he is describing contemporary knowledge of pre-Christian religion, which gives us a more direct look at its conception of sacred space than we would be able to find in Bede.
In this single sentence, even while employing praeteritio, Gildas manages to explain that these "divine honors"--some kind of animism--made the natural world "pernicious" (exitiabiles, which sometimes even means "deadly"), and that Christianity rendered the natural landscape "useful for human needs". The spaces that were formerly sacred and off limits were secularized and taken over by Christians who had more pertinent material needs in mind. In other words, the idea of finding divinity in the natural world is being denounced and discarded entirely. Of course, Gildas is here imposing his own judgment onto the shifting ideas of sanctity, but this change in attitude will be confirmed by another source through Bede.
This passage is preceded by a happier one where Gildas describes the natural world of Britain in the state that he finds it as a Christian. Employing secular but poetic language, it provides ample contrast with his image of dark and forgotten times.
Like a chosen bride arrayed in a variety of jewelry, the island is decorated with wide plains and agreeably set hills, excellent for vigorous agriculture, and mountains especially suited to varying the pasture for animals. Flowers of different hues underfoot made them a delightful picture. To water it, the island has clear fountains, whose constant flow drives before it pebbles white as snow, and brilliant rivers that glide with gentle murmur, guaranteeing sweet sleep for those who lie on their banks, and lakes flowing over with a cold rush of living water.
Gildas' idea of Britain is like a sort of Zion, a land of milk and honey that will subsequently be taken for granted and ruined by its ungrateful inhabitants. Moreover, it is a land that, "like a chosen bride", has been especially suited for human uses such as agriculture, herding, sleeping on riverbanks, and so forth. From that point on, the land of Britain figures not as a sacred space in Gildas' narrative but as a political one, a battleground on which Romano-Britons and Saxons squabble and fight.
Bede's Ecclesiastical History is permeated by similar material ideals. During the conversion of the English, Augustine arranges to meet King Æthelbert of Kent; but Æthelbert "took care that they should not meet in any building, for he held the traditional superstition that, if they practiced any magic art, they might deceive him and get the better of him as soon as he entered." Æthelbert's buildings did not have the benefit of electric lighting, so we can be sympathize with his worries over what pagan priests might hide in the corners of a dimly lit room. But here Bede draws a distinction between "divine and devilish power". Certainly the intention of this passage is that Christians would not stoop so low as to surprise the king with magic tricks, but the idea informing Bede's dismissal of the king's beliefs is that unlike "devilish" power which relies on "superstition", the power held by Christians works equally well in any space, indoors or outdoors.
During the story of the conversion of King Edwin of Northumbria, Bede demonstrates the consequences of this reimagining of sacred space more actively. This is not a matter of language choice but a full anecdote, and he must have relied on an older source for it, whether oral or written, since it took place 50 years before his birth. In the story, Edwin's head pagan priest Coifi confesses that the religion he practices seems to be useless, and Paulinus converts him to Christianity, after which Coifi symbolically destroys the idolatrous shrine.
Now a high priest of their religion was not allowed to carry arms or to ride except on a mare. So, girded with a sword, he took a spear in his hand and mounting the king's stallion he set off to where the idols were. The common people who saw him thought he was mad. But as soon as he approached the shrine, without any hesitation he profaned it by casting the spear which he held into it; and greatly rejoicing in the knowledge of the worship of the true God, he ordered his companions to destroy and set fire to the shrine and all he enclosures. 
It has been noted that Coifi's defamation of the shrine resembles how Odin, or Woden in Saxon England, traditionally thrusts a spear at his enemies on the battlefield, suggesting a mythological simile, but it is unclear if Bede knew this. The more explicit elements of Coifi's ride are also more powerful: the rules of the high priest are explicitly broken, and it is demonstrated to the common people that the old taboos and profanities are now meaningless. Part of this shift involves the destruction of the old sacred spaces, and shortly afterwards in the text Bede discusses the construction of a new church of stone elsewhere in Edwin's kingdom. In this story, at least, the move away from the pagan spaces is swift and complete.
Years before Coifi voluntarily destroyed his shrine, though, Pope Gregory I came to a different conclusion about the role pagan shrines should play in Christian Britain. This extraordinary letter speaks to some of the difficulties Christian missionaries encountered in the beliefs of the common people about their religious spaces, and outlines the compromises the Pope asked them to employ for an easy transition.
I have decided after long deliberation . that the idol temples of [the British] race should by no means be destroyed, but only the idols in them. Take holy water and sprinkle it in these shrines, build altars and place relics in them. For if the shrines are well built, it is essential that they should be changed . to the service of the true God. When the people see that their shrines are not destroyed they will be able to banish error from their hearts . [During religious feasts,] let them make themselves huts from the branches of trees around the churches which have been converted out of shrines . Do not let them sacrifice animals to the devil, but let them slaughter animals for their own food .
Again, we have some elements here of the nature of religion prior to the arrival of Christianity in Britain. It involved animal sacrifice, well-built shrines, and the construction of sacred huts out of raw forest material (the mention of tree branches in particular suggests that Gregory was not suggesting an innovation). And interestingly, Gregory does not demand a ban of these practices outright. Only two practices are too close to paganism for his comfort: idol worship, and the leaving the slaughtered animals as a sacrifice rather than eating them. This draws an interesting parallel to Gildas' trumping of the new secular uses Christians have brought to the island, but it's also possible Gregory was concerned only with preventing idol worship. In any case, the main message of this letter is that those places which were sacred to English pagans could still be maintained under Christian kingdoms. Only the understanding of what sanctity means would change.
The idea of sites of special power, demeaned by Gildas, did not go away entirely in Christian Britain. Instead, the nature of the sanctity changed. First, places were no longer chosen for natural beauty or rumors of magical power, but for their association with human beings, usually martyrs or saints. Second, it was no longer a matter of the location itself, but of objects that could be found there. An excellent example of this is the spot of the death of Oswald, as related by Bede. He explains that "people have often taken soil from the place where [Oswald's] body fell to the ground, have put it in water, and by its use brought great relief to the sick." Bede also relates that "many people are in the habit of cutting splinters from the wood of [a] holy cross" erected by Oswald, and one person was even healed by contact with the moss peeled from it. People are also healed in the place itself, but two of the three stories he passes on for these examples come from bringing relics to other places. In both instances, the idea of a holy space has been reified, so that the things that constitute the location (soil, moss, wood) work just as well as the location itself.
The creation or discovery of a sacred place, when it was not associated with relics, was closely linked to the founding of monasteries. In the Life of St. Columba, two different monks from Iona spend a good deal of time looking for a "desert in the ocean", that is to say, a place for hermitage and quiet contemplation. The Ecclesiastical History also contains the stories of monks who specifically seek out remote places for religious purposes, such as Bishop Aidan of Lindisfarne, who "often used to retire [to Farne Island] to pray in solitude and silence", or Bishop Cedd of London, who was granted a site of his choice by Prince Oethelwald:
Cedd chose for himself a site for the monastery amid some steep and remote hills which seemed better fitted for the haunts of robbers and the dens of wild beasts than for human inhabitation; so that, as Isaiah says, 'In the habitations where once dragons lay, shall be grass with reeds and rushes', that is, the fruit of good works shall spring up where once beasts dwelt or where men lived after the manner of beasts.
In the case of Cedd especially, Bede seems to view the founding of a remote monastery as a civilizing process as well as an opportunity for good works. He follows this passage by describing how Cedd "cleanse[s] the site . from the stain of former crimes"; not just the crimes of false religion, but anything people who "lived after the manner of beasts" might have done before a Christian presence arrived there. Even though the defining element of Cedd's site was its remoteness from civilization, which undoubtedly gave it some of the qualities of a "desert in the ocean", the land still needed to be purified before it could be used.
The Voyage of St. Brendan exemplifies the way early Christians thought of the natural world when Brendan and his fellows come upon a pleasant scene after suffering from hunger and thirst: "There before them was a sparkling spring set in the midst of a profusion of greenery. A rivulet led from the spring and it was full of every kind of fish pushing their way down to the sea. 'Brethren,' exclaimed the saint, 'what refreshment God has prepared for us after all our toil!'" The description of beauty on another island is quite similar: "Before them lay open country covered with apple trees laden with fruit. The monks ate as much as they wanted and drank deeply from the springs. The island was so wide that forty days' wandering still did not bring them to the farther shore." In this fantasy the motifs are reified: the places are special precisely because the things within them are useful for humans, and not so much in their own right. This is not a creation that the monks have to be careful around, but one that was prepared specifically for their pleasure.
Christian hagiographers show a corresponding interest in the utility of the natural world. Describing how Cuthbert was able to summon driftwood from the sea, Bede writes that "not only the inhabitants of air and ocean but the sea itself showed respect for the venerable old man." This verges on animism in its anthropomorphism of natural phenomena, but the intent is actually the opposite, as we learn in the next sentence that "it is hardly strange that the rest of creation should obey the wishes and commands of a man who has dedicated himself to the Lord's service". Bede's conclusion, though, is the most explicit statement of mastery over the British world: "We, on the other hand, often lose that dominion over creation which is ours by right by neglecting to serve its Creator." Bede's intent in this chapter of the Life of Cuthbert is to show that proper worship of God necessarily gives one the power to tame and rule over all natural things. Some of the miracles in Adamnan's Life of St. Columba parallel this dominion: Columba, "seeing [an apple tree's] boughs bearing to no purpose a load of fruit that injured rather than pleased those who tasted it", transforms the bitterness of the apples into sweetness, thus making them useful for human beings especially.
The Christian approach to space and the natural world is best summed up by Pope Gregory I, writing to Augustine: "For things are not to be loved for the sake of a place, but places are to be loved for the sake of their good things." Although Gregory is talking specifically about the utility of merging together practices from different churches to make the English church relevant to its own people, his statement of practicality and delocalization also applies to such practices as destroying pagan sites, converting "well built" pagan shrines into churches, and honoring the relics of saints rather than the places where the relics were left. In all of these cases, sacred things were no longer bound to their natural locations, so there is no evidence that the early Christians of Britain disagreed with Gregory's conclusion. In those instances where monks went looking specifically for a good place to pray or start a monastery, it is not the qualities of the space itself that are emphasized, but its relationship to civilization.
As Gildas describes, Christianity brought with it a set of assumptions about the universe that clashed with those of the pre-Christian Britons. It is difficult to say exactly what the pre-Christian Celts would have said about most of the approaches to the natural and sacred worlds outlined here, but I believe it unlikely that the pagan Britons would claim a "dominion over creation which is ours by right", as the Christians did. With the arrival of Christianity, the world became more "useful for human needs"; the religious, especially the saints among them, were able to extract an ever-larger bounty from it that was promised to them by God. 
 This is not to romanticize the pre-Christian Celts, about whom little can be reliably said, but Miranda J. Green, for example, concludes that an "essential animism . appears to have underpinned Celtic religion". "Natural cult foci" included "lakes, springs, and trees". M.J. Green, "The Gods and the Supernatural". In M.J. Green (ed.), The Celtic World. New York: Routledge, 1996. p. 465. See also J.-L. Brunaux, The Celtic Gauls: gods, rites, and sanctuaries. Trans. D. Nash. London: Seaby, 1988.
 Gildas. "On the Ruin of Britain" 4.2. Gildas: The Ruin of Britain and other works. Phillimore, 2002. 2nd edition. Trans. Michael Winterbottom.
 If Gildas had access to pagan sources and had looked very closely at them, he might have noticed that there were druids in Britain (Caesar's Gallic Wars, Tacitus' Annals) and that druids tended to gather in forests and worship oak trees (Lucan, Pliny's Nat.Hist.), but this shares nothing in common with his own description: forests instead of mountains, hills, and rivers, and religious specialists instead of generic "people". It seems more likely to suppose Gildas was aware of folk knowledge than to conclude that he spent his free time conjecturing from pagan histories.
 Gildas, "On the Ruin of Britain" 3.3-4.
 Bede. Hist. Eccl. I.24. Trans. Bertram Colgrave, 1969. New York: Oxford University Press, 1999.
 Bede, HE II.13.
 C.R. Davis, Beowulf and the Demise of Germanic Legend in England. New York: Garland, 1996. p. 35.
 J.M. Wallace-Hadrill, Bede's Ecclesiastical History of the English People: A Historical Commentary. Oxford: Oxford University Press, 1988. p. 72.
 Bede, HE I.30.
 Bede, HE III.9.
 Bede, HE III.2.
 Bede, HE III.16.
 Bede, HE III.23.
 "Voyage of St. Brendan". p.13 (of original manuscript). In D.H. Farmer (ed.), The Age of Bede. 1965. New York: Penguin Books, 2004.
 "Voyage of St. Brendan". p.28.
 Bede. "Life of Cuthbert". Ch. 21. In D.H. Farmer (ed.), The Age of Bede. 1965. New York: Penguin Books, 2004. Emphasis added.
 Adamnan. Life of Saint Columba. II.2,3. Ed. William Reeves. Edinburgh: Edmonston and Douglas, 1874. Emphasis added.
 Bede, HE I.27.
 And this belief was by no means limited to Britain. In early Christian Egypt, the mummified bodies of monks and nuns were believed to cause the annual, natural plant growths in the valley. c.f. Peter Brown, The Body and Society. New York: Columbia University Press, 1988. p. 437.