Concerning doubt as to the dedications of churches, about which among other things you have wished to consult us, you ought duly to hold to this which we have received as handed down to us from those who have gone before us; namely, that, as often as doubt is entertained as to the baptism or confirmation of any persons, as well as the consecration of churches, and there is no certain account to be given, either from writings or witnesses, as to whether persons have been baptized or confirmed, or whether churches have been consecrated, that such persons should be baptized and confirmed, and that such churches should be canonically dedicated, lest such doubt should become ruin to the faithful; inasmuch as what does not appear by certain proofs to have been duly done is not in such case done a second time.
This, divine grace supporting us, we desire so to hold; and we enjoin it on you, as you have requested, to hold and teach; and we wish not wantonly to break through, but faithfully to observe, what has been determined by holy Fathers before us. Wherefore we implore the mercy of our Redeemer to assist you with His grace, and give unto you to carry into effect what He has granted you to will, since in this matter the good gifts of retribution by so much the more accrue to us as the zeal of labour is increased.
But we decree that every one of those who have been faithfully taught, and already stand ineradicably planted with a firm root, shall observe his descent even to the seventh generation. And as long as they know themselves to be related to each other by affinity, let them not presume to approach the association of this union; nor is it lawful, or shall be lawfully for any Christian to marry a woman of his own kindred whom he has lived with as a wife, or whom he has stained by any unlawful pollution; since such intercourse is incestuous and abominable to God and to all good men.
But we read that it has long been determined by holy Fathers that incestuous persons are not to be reckoned under any title of wedlock. And so we desire not to be blamed by you or any other of the faithful in this matter, seeing that in our indulgence herein to the nation of the Angli we have acted, not as laying down a rule, but as taking thought lest they should leave imperfect the good which they had begun, etc.
Source: Register of Letters (New Advent)