28 When, therefore, he did not refuse to practise some of these Old Testament observances, he was not led by his compassion for Jews to feign this conformity, but unquestionably was acting sincerely; and by this course of action declaring his respect for those things which in the former dispensation had been for a time enjoined by God, he distinguished between them and the impious rites of heathenism. At that time, moreover, not with the subtlety of a deceiver, but with the love of one moved by compassion, he became to the Jews as a Jew, when, seeing them to be in error, which either made them unwilling to believe in Christ, or made them think that by these old sacrifices and ceremonial observances they could be cleansed from sin and made partakers of salvation, he desired so to deliver them from that error as if he saw not them, but himself, entangled in it; thus truly loving his neighbour as himself, and doing to others as he would have others do to him if he required their help—a duty to the statement of which our Lord added these words, “This is the law and the prophets.”
29. This compassionate affection Paul recommends in the same Epistle to the Galatians, saying: “If a man be overtaken in a fault, you which are spiritual restore such an one in the spirit of meekness; considering yourself, lest you also be tempted.” See whether he has not said, “Make yourself as he is, that you may gain him.” Not, indeed, that one should commit or pretend to have committed the same fault as the one who has been overtaken, but that in the fault of that other he should consider what might happen to himself, and so compassionately render assistance to that other, as he would wish that other to do to him if the case were his; that is, not with the subtlety of a deceiver, but with the love of one filled with compassion. Thus, whatever the error or fault in which Jew or Gentile or any man was found by Paul, to all men he became all things—not by feigning what was not true, but by feeling, because the case might have been his own, the compassion of one who put himself in the other's place—that he might gain all.
Source: Letters (New Advent)