1 Corinthians 9:20–22, Under the law. David Stern in his commentary asserts that this term means “under a legalistic perversion of the Torah”; i.e., that Torah here doesn’t mean Torah, but rather a skewed view of Torah as proffered by the Judaizers (i.e., those of the sect of the Pharisees or those who were under their influence who asserted that one needed to be circumcised and to keep the law of Moses in order to be saved, see Acts 15:1, 5).
In this passage, those who are “under the law” aren’t the Jews, since Paul already mentions them at the beginning of verse 20, but Gentiles who are living under a false notion that one can be saved by keeping the Torah.
Paul then goes on to describe the next group of Gentiles — “those who are without the Torah” (verse 21).
The third group of Gentiles to which Paul makes reference is “to the weak” (verse 22), who are those who are new to the faith and tend toward a more punctilious or legalistic obedience to the Torah.
In summary, Paul is saying that he’s sensitive to the spiritual paradigm of those to whom he is preaching the gospel, has learned to relate to all people from all backgrounds and, as such, has become all things to all men in an effort to save some.
Paul, at the same time, asserts that he is not “unTorahed” himself, but rather is “under or in subjection to the true Torah (as opposed to a legalistic perversion of the Torah) through his relationship with Yeshua (verse 21). This statement of Paul lines up perfectly with other statements he makes with regard to his pro-Torah beliefs and lifestyle (e.g., Rom 3:31; 7:12,14; Acts 21:24; 4:14; 25:8; 28:17; 1 Cor 7:19).