8 For this reason, from the books of the New Testament, except the figurative pre-significations used by our Lord, if you consider the life and manners of the Saints, their actions and sayings, nothing of the kind can be produced which should provoke to imitation of lying. For the simulation of Peter and Barnabas is not only recorded, but also reproved and corrected. For it was not, as some suppose, out of the same simulation that even Paul the Apostle either circumcised Timothy, or himself celebrated certain ceremonies according to the Jewish rite; but he did so, out of that liberty of his mind whereby he preached that neither are the Gentiles the better for circumcision, nor the Jews the worse.
Wherefore he judged that neither the former should be tied to the custom of the Jews, nor the Jews deterred from the custom of their fathers. Whence are those words of his: “Is any man called being circumcised let him not become uncircumcised. Is any called in uncircumcision? Let him not be circumcised. Circumcision is nothing, and uncircumcision is nothing, but the keeping of the commandments of God. Let every man abide in the same calling wherein he was called.” How can a man become uncircumcised after circumcision?
But let him not do so, says he: let him not so live as if he had become uncircumcised, that is, as if he had covered again with flesh the part that was bared, and ceased to be a Jew; as in another place he says, “Your circumcision has become uncircumcision.” And this the Apostle said, not as though he would compel either those to remain in uncircumcision, or the Jews in the custom of their fathers: but that neither these nor those should be forced to the other custom; and, each should have power of abiding in his own custom, not necessity of so doing.
For neither if the Jew should wish, where it would disturb no man, to recede from Jewish observances, would he be prohibited by the Apostle, since the object of his counselling to abide therein was that Jews might not by being troubled about superfluous things be hindered from coming to those things which are necessary to salvation. Neither would it be prohibited by him, if any of the Gentiles should wish to be circumcised for the purpose of showing that he does not detest the same as noxious, but holds it indifferently, as a seal, the usefulness of which had already passed away with time; for it did not follow that, if there were now no salvation to be had from it, there was destruction to be dreaded therefrom.
And for this reason, Timothy, having been called in uncircumcision, yet because his mother was a Jewess and he was bound, in order to gain his kindred, to show them that he had not learned in the Christian discipline to abominate the sacraments of the old Law, was circumcised by the Apostle; that in this way they might prove to the Jews, that the reason why the Gentiles do not receive them, is not that they are evil and were perniciously observed by the Fathers, but because they are no longer necessary to salvation after the advent of that so great Sacrament, which through so long times the whole of that ancient Scripture in its prophetical prefigurations did travail in birth withal.
For he would circumcise Titus also, when the Jews urged this, but that false brethren, privily brought in, wished it to be done to the intent they might have it to disseminate concerning Paul himself as a token that he had given place to the truth of their preaching, who said that the hope of Gospel salvation is in circumcision of the flesh and observances of that kind, and that without these Christ profits no man: whereas on the contrary Christ would nothing profit them, who should be circumcised because they thought that in it was salvation; whence that saying, “Behold, I Paul say unto you, that if you be circumcised, Christ shall profit you nothing.” Out of this liberty, therefore, did Paul keep the observances of his fathers, but with this one precaution and express declaration, that people should not suppose that without these was no Christian salvation.
Peter, however, by his making as though salvation consisted in Judaism, was compelling the Gentiles to judaize; as is shown by Paul's words, where he says, “Why do you compel the Gentiles to live as do the Jews?” For they would be under no compulsion unless they saw that he observed them in such manner as if beside them could be no salvation. Peter's simulation therefore is not to be compared to Paul's liberty. And while we ought to love Peter for that he willingly received correction, we must not bolster up lying even by the authority of Paul, who both recalled Peter to the right path in the presence of them all, lest the Gentiles through him should be compelled to judaize; and bore witness to his own preaching, that whereas he was accounted hostile to the traditions of the fathers in that he would not impose them on the Gentiles, he did not despise to celebrate them himself according to the custom of his fathers, and therein sufficiently showed that this has remained in them at the Coming of Christ; that neither to the Jews they are pernicious, nor to the Gentiles necessary, nor henceforth to any of mankind means of salvation.
Source: On Lying (New Advent)