What does Galatians 3:21 mean?
Paul is building his case for why the law cannot accomplish what faith in Christ can. Namely, he is referring to salvation—saving us from our sin. Paul has begun to answer the question "why the law," at all, then? Now he anticipates a different question from his readers: Is he suggesting that the law of Moses and the promises made to Abraham are in conflict?He answers emphatically, "Certainly not!" In the Greek it reads mē genoito, or "death to that idea!" In plain language, Paul really means it! Both the promises to Abraham and the law of Moses were given by God, after all. They are not opposed to each other; they simply serve different ultimate purposes.
If God had given a law designed to give life, then human beings could become righteous—"justified"—by following the law. The law cannot give life, however, because no human being has ever been able to keep it perfectly, aside from Christ Himself (Hebrews 4:15). Instead, as Paul will say in the following verse, the result of the law was to leave every human being in a prison of their own inescapable sinfulness. But, because of the promises to Abraham, as fulfilled in Jesus, that was not the end of the story.