Moving from "we" to a more personal "I," these verses come as the climax to Paul's narrative of his commissioning as a servant of the gospel. In thirteen references to "gospel" up to this point, Paul has heralded the gospel as the good news of the power of God's call to him and to all Christians through the grace shown in the death and resurrection of Jesus Christ, who has given himself to set people free from death and the present evil age (1:4, 15).
Now for the first time, Paul turns to talk of the good news in the new language of "justification" or "being made right with God." This witness is simply Paul's bold statement of faith ("we have come to believe") that is clearly grounded in his personal experience of God's call elaborated at some length in the preceding verses (1:11-2:14). In verse 16, he three times emphatically states the content of this belief, both positively and negatively and in reverse arrangement, the repetition marking these words as the significant capsule summary of the gospel that informs and shapes the whole of Galatians: "A person is justified not by works of the law but through faith in Jesus Christ. And we have come to believe in Christ Jesus so that we might be justified by faith in Christ, and not by doing the works of the law, because no one will be justified by the works of the law."
In verses 19-21, Paul then turns to describe the basis and the effects of this justifying grace of God. In what is certainly a reference to baptism (see Galatians 3:27 and the more specific development of this idea in Romans 6:1-11), justification comes through being joined to the crucifixion of Christ. Being joined to the death and resurrection of Christ means to experience in the flesh that "new creation" (Galatians 6:15) in which Paul and every Christian already live--not in the self, but by faith in the Son of God, "who loved me and gave himself for me" (verse 20).
15 We ourselves are Jews by birth and not Gentile sinners;
16yet we know that a person is justified* not by the works of the law but through faith in Jesus Christ.* And we have come to believe in Christ Jesus, so that we might be justified by faith in Christ,* and not by doing the works of the law, because no one will be justified by the works of the law.
17But if, in our effort to be justified in Christ, we ourselves have been found to be sinners, is Christ then a servant of sin? Certainly not!
18But if I build up again the very things that I once tore down, then I demonstrate that I am a transgressor.
19For through the law I died to the law, so that I might live to God. I have been crucified with Christ;
20and it is no longer I who live, but it is Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God,* who loved me and gave himself for me.
21I do not nullify the grace of God; for if justification* comes through the law, then Christ died for nothing.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org