A man is not a Jew because he is one outwardly, nor is circumcision only outward and physical.
28. For a Jew is not he, etc. The meaning is, that a real Jew is not to be ascertained, either by natural descent, or by profession, or by an external symbol; that the circumcision which constitutes a Jew, does not consist in an outward sign only, but that both are inward.
In the latter part of the chapter the apostle directs his discourse more closely to the Jews, and shows what sins they were guilty of, notwithstanding their profession and vain pretensions. He had said (Rom 2:13) that not the hearers but the doers of the law are justified; and he here applies that great truth to the Jews. Observe, I.
Commenting on Romans 2:17-29
Verse 28. For he is not a Jew, etc. He who is merely descended from Abraham, and is circumcised, and externally conforms to the law only, does not possess the true character, and manifest the true spirit, contemplated by the separation of the Jewish people. Their separation required much more. Neither is that circumcision. etc.