John Calvin on Romans 2:28

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. And what he subjoins with regard to 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 fore-skin was cut off, not indeed as the small corruption of one part, but as that of the whole nature. Circumcision then signified the mortification of the whole flesh.