7 And further, analogy, whereby the agreement of both Testaments is plainly seen, why shall I say that all have made use of, to whose authority they yield; whereas it is in their power to consider with themselves, how many things they are wont to say have been inserted in the divine Scriptures by certain, I know not who, corrupters of truth? Which speech of theirs I always thought to be most weak, even at the time that I was their hearer: nor I alone, but you also, (for I well remember,) and all of us, who essayed to exercise a little more care in forming a judgment than the crowd of hearers.
But now, after that many things have been expounded and made clear to me, which used chiefly to move me: those I mean, wherein their discourse for the most part boasts itself, and expatiates the more freely, the more safely it can do so as having no opponent; it seems to me that there is no assertion of theirs more shameless, or (to use a milder phrase) more careless and weak than that the divine Scriptures have been corrupted; whereas there are no copies in existence, in a matter of so recent date, whereby they can prove it.
For were they to assert, that they thought not that they ought thoroughly to receive them, because they had been written by persons, who they thought had not written the truth; any how their refusal would be more right, or their error more natural. For this is what they have done in the case of the Book which is inscribed the Acts of the Apostles. And this device of theirs, when I consider with myself, I cannot enough wonder at. For it is not the want of wisdom in the men that I complain of in this matter, but the want of ordinary understanding. For that book has so great matters, which are like what they receive, that it seems to me great folly to refuse to receive this book also, and if any thing offend them there to call it false and inserted.
Or, if such language is shameless, as it is why in the Epistles of Paul, why in the four books of the Gospel, do they think that they are of any avail, in which I am not sure but that there are in proportion many more things, than could be in that book, which they will have believed to have been interpolated by falsifiers. But forsooth this is what I believe to be the case, and I ask of you to consider it with me with as calm and serene a judgment as possible. For you know that, essaying to bring the person of their founder Manichæus into the number of the Apostles, they say that the Holy Spirit, Whom the Lord promised His disciples that He would send, has come to us through him.
Therefore, were they to receive those Acts of the Apostles, in which the coming of the Holy Spirit is plainly set forth, they could not find how to say that it was interpolated. For they will have it that there were some, I know not who, falsifiers of the divine Books before the times of Manichæus himself; and that they were falsified by persons who wished to combine the Law of the Jews with the Gospel. But this they cannot say concerning the Holy Spirit, unless haply they assert that those persons divined, and set in their books what should be brought forward against Manichæus, who should at some future time arise, and say that the Holy Spirit had been sent through him. But concerning the Holy Spirit we will speak somewhat more plainly in another place. Now let us return to my purpose.
Source: On the Profit of Believing (New Advent)