27 Where then is boasting? It is excluded. By what kind of law? Of works? No, but by a law of faith. 28 For we maintain that a man is justified by faith apart from works of the Law. 29 Or is God the God of Jews only? Is He not the God of Gentiles also? Yes, of Gentiles also, 30 since indeed God who will justify the circumcised by faith and the uncircumcised through faith is one (Romans 3).
No boasting, period. No boasting in works either by Jews or Gentiles. But did you notice that Jews are justified by faith, while Gentiles are justified through faith? How is that? What does this mean? Well, the word by carries the idea of separation, that faith separates the Jew from the Law so that He is no longer bound by it. Since Gentiles did not have the Law in the first place, then no separation was needed — that is why they are justified through faith, which acted as a channel for or a means to it, which is the meaning the word through conveys.
Paul makes this distinction to emphasize separation from the Mosaic Law for the Jews, and a channel for or means to salvation for the Gentiles. Yet, as a Gentile, my faith also separates me from having to fulfill a general law of works, and likewise, the faith of Jews in Christ is also a channel for or a means to their salvation as well. In either case, though, there is no boasting in ourselves, for it is all by the grace of God — thus any boasting must be in Him and Him alone.
Lord, we thank You for this faith that by grace You have given us! Thank You that by it and through it we have been saved. May we boast in You This Day — all day long! Amen.