Galatians 2:21

Frustrate. Or, “thwart,” “nullify,” “make void.” To return to the legal system of righteousness by works would nullify all that has been received through the grace of God, by faith in Jesus Christ. Paul refuses even for one moment to concede to the demands of those who, like Peter and the Judaizers, insist on a return to the works of the law as a requirement for justification. This must be considered as a reproof to the attitude taken by Peter (see on v. 15). Thus Paul’s evident conclusion is that all who, like Peter, separate themselves from the Gentile Christians, will be frustrating the grace of God.

If righteousness come. If grace is not needed the gospel has no value or appeal. In discussing the righteousness which Abraham received through faith, Paul argues that the rewards of one who works are not a favor or a gift; they belong to him, as the result of his own efforts (see Rom. 4:4, 5). Now if, by the works of the law, one may obtain all the benefits that come through the gospel, the plan for man’s redemption through Jesus Christ has been made unnecessary. The logic of this conclusion must have been apparent to Peter and to all who were following his example.