“For no one is a Jew who is merely one outwardly, nor is circumcision outward and physical. But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter. His praise is not from man but from God.” -Romans 2:28–29
Physical circumcision is the cutting off of the foreskin, not as representing the corruption of one part of the body, but as a symbol of the corruption of the whole nature. Therefore, circumcision is really “a matter of the heart,” symbolizing the mortification of the flesh, of the whole Adamic nature (Cf. 2 Corinthians 3:6 and Jeremiah 31:33)
To be a Jew, was to be the people of God, inwardly. The Spirit refers to the spiritual nature of the rite; whereas, the letter refers to the outward rite alone, as that which lack the necessary inward piety.
The true Jew seeks the praise and affirmation of God, who alone sees the inward, and not the praise of man, who is limited to only seeing the outward.
“But the Lord said to Samuel, “Do not look on his appearance or on the height of his stature, because I have rejected him. For the Lord sees not as man sees: man looks on the outward appearance, but the Lord looks on the heart.”” -1 Samuel 16:7