John Calvin Commentary Romans 2:28

"For he is not a Jew who is one outwardly; neither is that circumcision which is outward in the flesh:" — Romans 2:28 (ASV)

For a Jew is not he, etc. The meaning is that a real Jew is not to be ascertained either by natural descent, by profession, or by an external symbol; that the circumcision which constitutes a Jew does not consist only in an outward sign, but that both are inward.

And what he adds regarding true circumcision is taken from various passages of Scripture, and even from its general teaching; for the people are everywhere commanded to circumcise their hearts, and it is what the Lord promises to do.

The foreskin was cut off, not indeed as the small corruption of one part only, but as the corruption of the whole nature. Circumcision then signified the mortification of the whole flesh.