Tertullian

Now it is quite within the purpose of the God of the law that circumcision should be that of the heart, not in the flesh; in the spirit, and not in the letter. Since this is the circumcision recommended by Jeremiah: "Circumcise (yourselves to the Lord, and take away) the foreskins of your heart; " and even of Moses: "Circumcise, therefore, the hardness of your heart," -the Spirit which circumcises the heart will proceed from Him who prescribed the letter also which clips the flesh; and "the Jew which is one inwardly" will be a subject of the self-same God as he also is who is "a Jew outwardly; " because the apostle would have preferred not to have mentioned a Jew at all, unless he were a servant of the God of the Jews.