Albert Barnes Commentary 1 Corinthians 9:20

"And to the Jews I became as a Jew, that I might gain Jews; to them that are under the law, as under the law, not being myself under the law, that I might gain them that are under the law;" — 1 Corinthians 9:20 (ASV)

And unto the Jews. In this verse, and the two following, Paul states more fully the conduct which he had displayed, and to which he refers in 1 Corinthians 9:19. He had shown this conduct to all classes of men. He had preached much to his own countrymen, and had demonstrated these principles there.

I became as a Jew. I complied with their rites, customs, prejudices, as far as I could with a good conscience. I did not needlessly offend them. I did not attack and oppose their views when there was no danger that my conduct would be misunderstood. For a full illustration of Paul's conduct in this respect, and the principles that influenced him, see Acts 16:3; Acts 18:18; Acts 21:21; also Acts 21:22-27; Acts 23:1; also Acts 23:5-6.

To those that are under the law. This I understand as another form of saying that he conformed to the rites, customs, and even prejudices of the Jews. The phrase, "under the law," means undoubtedly the law of Moses; and he probably refers here particularly to those Jews who lived in the land of Judea, as being more immediately and entirely under the law of Moses than those who lived among the Gentiles.

As under the law. That is, I conformed to their rites and customs as far as I could do it. I did not violate them unnecessarily. I did not disregard them for the purpose of offending them, nor refuse to observe them when it could be done with a good conscience. There can be no doubt that Paul, when he was in Judea, submitted himself to the laws, and lived in conformity with them.

That I might gain. That I might obtain their confidence and affection. That I might not outrage their feelings, excite their prejudices, and provoke them to anger; and that I might thus have access to their minds, and be the means of converting them to the Christian faith.