12 But now, that as bearing with the infirmity of men he did this, let us hear what follows: “For though I be free from all men, yet have I made myself servant unto all, that I might gain the more. To them that are under the law, I became as under the law, that I might gain them that are under the law; to them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.” Which thing he did, not with craftiness of simulation, but with mercy of compassion with others; that is, not as if to feign himself a Jew, as some have thought, in that he observed at Jerusalem the things prescribed by the old law. For he did this in accordance with his free and openly declared sentence, in which he says, “Is any called being circumcised?
Let him not become uncircumcised.” That is, let him not so live, as though he had become uncircumcised, and covered that which he had laid bare: as in another place he says, “Your circumcision has become uncircumcision.” It was in accordance then with this his sentence, in which he says, “Is any called being circumcised? Let him not become uncircumcised. Is any called in uncircumcision? Let him not be circumcised;” that he did those things, in which, by persons not understanding and not enough attending, he has been accounted to have feigned.
For he was a Jew, and was called being circumcised; therefore he would not become uncircumcised; that is, would not so live as if he had not been circumcised. For this he now had in his power to do. And “under” the law, indeed, he was not as they who servilely wrought it; but yet “in” the law of God and of Christ. For that law was not one, and the law of God another, as accursed Manicheans are wont to say. Otherwise, if when he did those things he is to be accounted to have feigned, then he feigned himself also a pagan, and sacrificed to idols, because he says that he became to those without law, as without law.
By whom, doubtless, he would have us to understand no other than Gentiles whom we call Pagans. It is one thing therefore to be under the law, another in the law, another without law. “Under the law,” the carnal Jews; “in the law,” spiritual men, both Jews and Christians; (whence the former kept that custom of their fathers, but did not impose unwonted burdens upon the believing Gentiles; and therefore they also were circumcised;) but “without law,” are the Gentiles which have not yet believed, to whom yet the Apostle testifies himself to have become like, through sympathy of a merciful heart, not simulation of a changeable exterior; that is, that he might in that way succor carnal Jew or Pagan, in which way himself, if he were that, would have wished to be succored: bearing, to wit, their infirmity, in likeness of compassion, not deceiving in fiction of lying; as he straightway goes on, and says, “I became to the weak as weak, that I might gain the weak.” For it was from this point that he was speaking, in saying all those other things.
As then, that he became to the weak as weak, was no lie; so all those other things above rehearsed. For what does he mean his weakness towards the weak to have been, but that of suffering with them, insomuch that, lest he should appear to be a seller of the Gospel, and by falling into an ill suspicion with ignorant men, should hinder the course of God's word, he would not accept what by warrant of the Lord was his due? Which if he were willing to accept, he would not in any wise lie, because it was truly due to him; and for that he would not, he did not in any wise lie.
For he did not say, it was not due; but he showed it to be due, and that being due he had not used it, and professed that he would not at all use it, in that very thing becoming weak; namely, in that he would not use his power; being, to wit, with so merciful affection endued, that he thought in what way he should wish to be dealt withal, if himself also were made so weak, that possibly, if he should see them by whom the Gospel was preached to him, accepting their charges, he might think it a bringing of wares to market, and hold them in suspicion accordingly.
Source: On the Work of Monks (New Advent)