The following is an excerpt from The MacArthur New Testament Commentary on 1 Corinthians 9.

To the weak I became weak, that I might win the weak; I have become all things to all men, that I may by all means save some. (9:22)

To the weak I became weak. Third, Paul was willing to identify with those, whether Jew or Gentile, who did not have the power of understanding to grasp the gospel. When among those who were weak he acted weak. He stooped to the level of their weakness of comprehension. To those who needed simple or repeated presentations, that is what he gave them. No doubt he demonstrated that kind of consideration in the case of the Corinthians themselves (cf. 2:1–5). His purpose was to win them to salvation.

In summary, Paul became all things to all men, that he might by all means save some. He did not compromise the gospel. He would not change the least truth in the least way in order to satisfy anyone. But he would condescend in any way for anyone if that would in any way help bring him to Christ. He would never set aside a truth of the gospel, but he would gladly restrict his liberty in the gospel. He would not offend Jew, Gentile, or those weak in understanding.

If a person is offended by God’s Word, that is his problem. If he is offended by biblical doctrine, standards, or church discipline, that is his problem. That person is offended by God. But if he is offended by our unnecessary behavior or practices—no matter how good and acceptable those may be in themselves—his problem becomes our problem. It is not a problem of law but a problem of love, and love always demands more than the law. “Whoever slaps you on your right cheek, turn to him the other also. And if anyone wants to sue you, and take your shirt, let him have your coat also. And whoever shall force you to go one mile, go with him two” (Matt. 5:39–41).

Paul’s life centered in living out the gospel and in preaching and teaching the gospel. Nothing else was of any concern to him. I do all things for the sake of the gospel. His life was the gospel. He therefore set aside anything that would hinder its power and effectiveness.

Fellow partaker (sunkoinonos) refers to joint participation, joint sharing. The idea here is that Paul wanted everyone else to be a fellow partaker with him in the benefits and blessing of the gospel. He wanted them to be with him in the family of God.