43 So great blindness, moreover, has occupied men's minds, that to them it is too little if we pronounce some lies not to be sins; but they must needs pronounce it to be sin in some things if we refuse to lie: and to such a pass have they been brought by defending lying, that even that first kind which is of all the most abominably wicked they pronounce to have been used by the Apostle Paul. For in the Epistle to the Galatians, written as it was, like the rest, for doctrine of religion and piety, they say that he has told a lie, in the passage where he says concerning Peter and Barnabas, “When I saw that they walked not uprightly according to the truth of the Gospel.” For, while they wish to defend Peter from error, and from that pravity of way into which he had fallen; the very way of religion in which is salvation for all men, they by breaking and mincing the authority of the Scriptures do endeavor themselves to overthrow.
In which they do not see that it is not only lying, but perjury that they lay to the charge of the Apostle in the very doctrine of piety, that is, in an Epistle in which he preaches the Gospel; seeing that he there says, before he relates that matter, “What I write unto you, behold, before God, I lie not.” But it is time that we set bounds to this disputation: in the consideration and treatment whereof altogether there is nothing more meet to be, before all else, borne in mind and made our prayer, than that which the same Apostle says: “God is faithful, Who will not suffer you to be tempted above that you are able to bear, but will with the temptation make also a way to escape, that you may be able to bear it.”
Source: On Lying (New Advent)