Paul begins with a point that all Jewish Christians acknowledged and affirmed. The subject of the verb know in verse 16 is given in verse 15: we who are Jews by birth and not "Gentile sinners" know . . . Paul is developing the same kind of argument that he used against Peter in verse 14. If Peter, being a Jew, lived as a Gentile and not as a Jew, how could he require Gentiles to live as Jews? Paul continues this line of reasoning by saying that if we who are Jews by birth and not "Gentile sinners" (v. 15) know that we, too, believe in Christ Jesus in order to be justified by faith in Christ and not by observing the law (v. 16), then we must recognize that "Gentile sinners" can be justified only by faith in Christ Jesus, not by observing the law.

Jews considered themselves to be God's covenant people; Gentiles were considered to be sinners because they were not part of that covenant people. But Jewish Christians recognized that God's judicial pronouncement that someone is part of the covenant people is not based on this Jew-Gentile distinction. The covenant is no longer conceived in nationalistic terms. Even though as Jews they claimed a privileged status, now as Jewish Christians they knew that only those who believe in Jesus Christ are justified, declared by God to belong to the covenant family.

In the context of Paul's account of the disputes at Jerusalem (vv. 3-6) and Antioch (vv. 11-14), the phrase observing the law refers to circumcision and the Jewish purity laws. The Jewish people were identified by their observance of these laws. So what Paul is denying in this context is that identification with the Jewish people through observance of these distinctively Jewish practices is not the basis of membership in the covenant people of God. Paul is appealing to the common affirmation of Jewish Christians themselves that believing in Christ Jesus, not following "Jewish customs" (v. 14), is the basis of being justified. In the final clause of verse 16, Paul paraphrases Psalm 143:2 to show the universal scope of this affirmation of Jewish Christians: by observing the law no one will be justified. The main emphasis of Paul's argument here is that faith in Jesus Christ replaces and excludes Jewishness as the determining criterion for belonging to the people of God.

We will see that this point of agreement was confirmed by the experience of the Gentile believers at Galatia (3:1-5). Just as Jewish Christians came to know that they were justified by faith in Christ, and not by any Jewish privileges or customs, so Gentile believers experienced the Spirit, the sign of covenant blessing (3:14), by their faith response to the message of Christ, not by their acceptance of circumcision, Jewish food regulations or sabbath observance (see 4:10). The exclusion of observing the law as a basis for justification is developed more fully in 3:10-14. There the law is expanded to include all the works commanded by the Mosaic law.