Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
The aforesaid envoys having departed, the blessed Father Gregory sent after them a letter worthy to be recorded, wherein he plainly shows how carefully he watched over the salvation of our country. The letter was as follows:
"To his most beloved son, the Abbot Mellitus; Gregory, the servant of the servants of God. We have been much concerned, since the departure of our people that are with you, because we have received no account of the success of your journey. Howbeit, when Almighty God has led, you to the most reverend Bishop Augustine, our brother, tell him what I have long been considering in my own mind concerning the matter of the English people; to wit, 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 water be consecrated and sprinkled in the said temples, let altars be erected, and relics placed there. For if those temples are well built, it is requisite that they 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 freely resort to the places to which they have been accustomed. And because they are used to slaughter many oxen in sacrifice to devils, some solemnity must be given them in exchange for this, as that on the day of the dedication, or the nativities of the holy martyrs, whose relics are there deposited, they should build themselves huts of the boughs of trees about those churches which have been turned to that use from being temples, and celebrate the solemnity with religious feasting, and no more offer animals to the Devil, but kill cattle and glorify God in their feast, and return thanks to the Giver of all things for their abundance; to the end that, whilst some outward gratifications are retained, they may the more easily consent to the inward joys. For there is no doubt that it is impossible to cut off every thing at once from their rude natures; because he who endeavours to ascend to the highest place rises by degrees or steps, and not by leaps. Thus the Lord made Himself known to the people of Israel in Egypt; and yet He allowed them the use, in His own worship, of the sacrifices which they were wont to offer to the Devil, commanding them in His sacrifice to kill animals, to the end that, with changed hearts, they might lay aside one part of the sacrifice, whilst they retained another; and although the animals were the same as those which they were wont to offer, they should offer them to the true God, and not to idols; and thus they would no longer be the same sacrifices. This then, dearly beloved, it behoves you to communicate to our aforesaid brother, that he, being placed where he is at present, may consider how he is to order all things. God preserve you in safety, most beloved son.
"Given the 17th of June, in the nineteenth year of the reign of our most religious lord, Mauritius Tiberius Augustus, the eighteenth year after the consulship of our said lord, and the fourth indiction."