Having answered the Judaizers’ criticisms of his gospel (and of him) in Galatians 1:11-2:14, Paul presents in 2:15-21 the theme of the letter–justification by faith, anticipating his fuller consideration in chapters 3 and 4 of that central doctrine in Pauline (and Christian) theology.
Because the passage seems to be a continuation of Paul’s address to Peter that began in 2:14 (“If you, though a Jew, live like a Gentile and not like a Jew, how can you force the Gentiles to live like Jews?”), some Bible versions carry the address on to the end of verse 16 or to the end of the chapter. However because of the passage’s content, I placed it at the beginning of the doctrinal section in the outline of Galatians which I provided in [Galatians — the Magna Carta of Christian Liberty]. If Paul wrote Galatians when he was at Antioch of Syria after his first missionary journey (I think he did, but many scholars think that he wrote it during his third missionary journey), the passage is Paul’s first systematic treatment of justification by faith.
15 We ourselves are Jews by birth and not Gentile sinners; 16 yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified. (ESV; all Biblical quotations are from the ESV.)
The phrase “faith in Jesus Christ” is translated “the faith of Jesus Christ” in the King James Version and can also be translated “the faithfulness of Jesus Christ,” but it is translated “faith in Jesus Christ” in all the modern versions of the Bible that I consulted and I think that that rendition makes more sense in the context. Thus, Paul begins by observing that even believers of Jewish birth (such as Peter and himself) knew that justification is by faith in Jesus Christ and not by works of the law.
– Justification is God’s act of declaring people righteous. It is His act of declaring, not of making people righteous, but ethical changes follow as a result of a person’s being justified by faith and indwelt by the Holy Spirit.
– Faith in Jesus Christ is trust in the gospel message concerning Jesus Christ. Faith is the means by which justification is received, not its source, which is Jesus Christ.
– Works of the law are deeds done in obedience to Mosaic law. Interpretations of what Paul meant by “works of the law” range from its referring only to those parts of Mosaic law setting Jews apart from others–circumcision, food laws, and Sabbath observance–to its including all good deeds. I think that he was referring to the law of Moses as a whole. However, I don’t think that he was opposed to the law itself, but to the legalistic use of the law as a way of winning favour with God.
Paul’s point seems to be that if the Jewish Christians realize that justification is by faith in Jesus Christ and not by works of the law, then they shouldn’t be trying to force “Gentile sinners” to submit to the law of Moses.
17 But if, in our endeavor to be justified in Christ, we too were found to be sinners, is Christ then a servant of sin? Certainly not! 18 For if I rebuild what I tore down, I prove myself to be a transgressor.
Apparently, the criticism was made that attributing justification to faith alone would lead to believers abandoning moral standards and living licentiously. To the question of whether this made Christ a promoter of sin, Paul responds, “Certainly not!” (Because verse 17 is obscure, there are other quite different interpretations of it; for example, since being justified by faith involves abandoning the law and abandoning the law is sin, is Christ, for whom we abandoned the law, therefore the promoter of sin?) Rather, anyone who returns to the law after being justified by faith denies what Christ did for him and makes himself a sinner again.
19 For through the law I died to the law, so that I might live to God. 20 I have been crucified with Christ. It is no longer I who live, but 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.
Paul describes as dying to the law his abandoning the law as a means of obtaining acceptance by God. He observes that it was through the law itself that he died to it (he doesn’t explain the connection but possibly had in mind the idea of Galatians 3:19-25 or of Romans 7:4-6) and that his purpose in dying to the law was that he might live unto God. Paul could do this because God had so united him with Christ that he participated in Christ’s death to the law and resurrection to new life (compare Romans 6:4, “We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life”). Now Christ, through the Holy Spirit, lives in him so that he is not only justified but also morally and spiritually guided by Christ.
21 I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.
Having answered the objections of the Judaizers, Paul states emphatically, “I do not nullify the grace of God,” implying that they were doing so. (Paul may be responding to an objection by them that he is nullifying the grace of God in His providing the law as a means of being accepted by Him). If people could be justified by observing the law, then there was no need for Christ to die on their behalf. However Christ did die on their behalf, and so they no longer had to observe the law to be justified. Indeed, if they insisted that it was necessary to observe the law to be justified, they nullified what Christ had done for them and he had died for nothing.
Paul doesn’t record how Peter responded to what Paul said to him. (His not doing so suggests that Peter didn’t give in to Paul’s rebuke. However, Peter’s address to the Jerusalem church afterwards [Acts 15:7-11] indicates that by that time he supported Paul’s position.) However, the important thing for us today is how we respond to it. Are you trying to mix law and grace as the Judaizers did, or are you trusting completely in what Jesus Christ did for your salvation?
Bob's Corner 
Good ending question! I think it’s difficult to live for God and not sometimes slip into legalism, but equally difficult to live in this world and not sometimes slip into sin. How believers I wonder know what it means to live by pure grace?
“Good ending question!” Thanks, Allison.
“I think it’s difficult to live for God and not sometimes slip into legalism, but equally difficult to live in this world and not sometimes slip into sin.” I know that it’s difficult for me and suspect that it’s also difficult for other Christians.