Gregory the Great: Instructions to the Missionaries
The Letter to Mellitus of 601
When Almighty God shall bring you to the most reverend Bishop Augustine, our brother, tell him what I have, after mature deliberation on the affairs of the English, determined upon, namely, that the temples of the idols in that nation ought not to be destroyed, but let the idols that are in them be destroyed; let holy water be made and sprinkled in thesaid temples - let altars be erected, and relics placed. For if those temples are well built, it is requisite that the be converted from the worship of devils to the service of the true God; that the nation, seeing that their temples are not destroyed, may remove error from their hearts and, knowing and adoring the true God, may the more familiarly resort tothe places to which they have been accustomed.
And because they have been used to slaughter many oxen in the sacrifices to devils, some solemnity must be substituted for them on this account, as, for instance, that on the day of the dedication, or of the nativities of the holy martyrs whose relics are there deposited, they may build themselves huts of the boughs of trees about those churches which have been turned to that use from temples, and celebrate the solemnity with religious feasting, no more offering beasts to the devil, but killing cattle to the praise of God in their eating, and returning thanks to the Giver of all things for their sustenance; to the end that, whilst some outward gratifications are permitted them, they may the more easily consent to thee inward consolations of the grace of God.
For there is no doubt that it is impossible to efface every thing at once from their obdurate minds., because he who endeavors to ascend to the highest place rises by degrees or steps and not by leaps. This the Lord made himself known to the people of Israel in Egypt: and yet he allowed them to use the sacrifices which they were wont to offer to the devil in his own worship, commanding them in his sacrifice to kill beasts to the end that, changing their hearts they mad lay aside one part of the sacrifice whilst retained another: that whilest they offered the same beasts which they were wont to offer, they should offer them to God, and not to idols, and thus they would no longer be the same sacrifices.