Paul focuses on the boastful Jew: 'If thou be a breaker of the law, thy circumcision is made uncircumcision. For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.'  Romans 2: 25, 28-29

Paul brings up this strange concept of circumcision of the heart. Whereas the old circumcision marked the covenant people of God, new circumcision of the heart now marks his covenant people. A circumcised heart is not an outward ordinance but an inward manifestation. 

Paul clarifies that being chosen of God as the Jews thought, is not by birth. Being chosen of God comes through circumcision of the heart. Rather than write upon tablets of stone, God writes upon the heart,  'I will put my law into their mind and write it on their hearts,' Jeremiah 31:33 and gives us a new heart, 'A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.' Ezekiel 36:26  

This is the new covenant promise. Both Jews and Gentiles who put their faith in Christ, experience circumcision of the heart, and are part of the body of Christ.