4 Moreover, a man of your talent and learning easily perceives how different from these metaphorical expressions is the statement of the apostle, “When I saw that they walked not uprightly, according to the truth of the gospel, I said unto Peter before them all, If you, being a Jew, livest after the manner of the Gentiles, and not as do the Jews, why do you compel the Gentiles to live as do the Jews?” Here there is no obscurity of figurative language; these are literal words of a plain statement. Surely, in addressing persons “of whom he travailed in birth till Christ should be formed in them,” and to whom, in solemnly calling God to confirm his words, he said: “The things which I write unto you, behold, before God, I lie not,” the great teacher of the Gentiles affirmed in the words above quoted either what was true or what was false; if he said what was false, which God forbid, you see the consequences which would follow; and Paul's own assertion of his veracity, together with the example of wondrous humility in the Apostle Peter, may warn you to recoil from such thoughts.
Source: Letters (New Advent)