What does 1 Corinthians 9:21 mean?
Paul is describing how he, as a free person, has placed himself under the authority of all men in hopes of winning some to faith in Christ. In the previous verse, he wrote that he became as a Jewish law-follower to win some of them. Now he writes that he became as one outside of the law in order to win some of those people, as well.Paul has written much in Galatians and Romans about freedom in Christ, including the freedom from following the law of Moses (Romans 10:4). Paul continued to participate in aspects of Jewish religious life, but he also demonstrated freedom in Christ to the Gentiles. This came by participating in some things forbidden for followers of the law of Moses. Beyond that, he made clear to Gentiles that it was unnecessary for them to follow the law in order to be acceptable to God. Salvation is only possible through faith in Christ.
For one who had been a Pharisee and a "Hebrew of Hebrews" (Philippians 3:5–6) to live outside the law of Moses required enormous confidence in the grace of God through faith in Christ. Paul's demonstration of this confidence showed the Gentiles the sincerity of his teaching.
Still, Paul adds that he remained under the authority of law of Christ; the law of self-sacrificing love for God and others (Mark 12:30–31). Every Christian is under that law. As such, it is more important to consider the needs of others than to demand whatever we think we are entitled to.