And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the {p} law, as under the law, that I might gain them that are under the law;

(p) The word law in this place must be limited to the ceremonial Law.

People's New Testament

9:20 Unto the Jews I became as a Jew. With Jews he lived as a Jew in order to reach them. He observed their distinctions of meats, kept feasts (Ac 18:21), and circumcised Timothy (Ac 16:3). He observed the law to reach those who kept law.

Wesley's Notes

9:20 To the Jews I became as a Jew - Conforming myself in all things to their manner of thinking and living, so far as; I could with innocence. To them that are under the law - Who apprehend themselves to be still bound by the Mosaic law. As under the law - Observing it myself, while I am among them. Not that he declared this to be necessary, or refused to converse with those who did not observe it. This was the very thing which he condemned in St. Peter, Gal 2:14.

Jamieson-Fausset-Brown Bible Commentary

20. I became as a Jew-in things not defined by the law, but by Jewish usage. Not Judaizing in essentials, but in matters where there was no compromise of principle (compare Ac 16:3; 21:20-26); an undesigned coincidence between the history and the Epistle, and so a sure proof of genuineness.

to them that are under the law, as under the law-in things defined by the law; such as ceremonies not then repugnant to Christianity. Perhaps the reason for distinguishing this class from the former is that Paul himself belonged nationally to "the Jews," but did not in creed belong to the class of "them that are under the law." This view is confirmed by the reading inserted here by the oldest manuscripts, versions, and Fathers, "not being (that is, parenthetically, 'not that I am') myself under the law."

Matthew Henry's Concise Commentary

9:15-23 It is the glory of a minister to deny himself, that he may serve Christ and save souls. But when a minister gives up his right for the sake of the gospel, he does more than his charge and office demands. By preaching the gospel, freely, the apostle showed that he acted from principles of zeal and love, and thus enjoyed much comfort and hope in his soul. And though he looked on the ceremonial law as a yoke taken off by Christ, yet he submitted to it, that he might work upon the Jews, do away their prejudices, prevail with them to hear the gospel, and win them over to Christ. Though he would transgress no laws of Christ, to please any man, yet he would accommodate himself to all men, where he might do it lawfully, to gain some. Doing good was the study and business of his life; and, that he might reach this end, he did not stand on privileges. We must carefully watch against extremes, and against relying on any thing but trust in Christ alone. We must not allow errors or faults, so as to hurt others, or disgrace the gospel.