The Mark

Romans 2:25-29 ESV 

For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision. So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision? Then he who is physically uncircumcised but keeps the law will condemn you who have the written code and circumcision but break the law. 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.

In the end, circumcision is the mark of god on His people. The mark of the Old Covenant was on the outward flesh of His elect people. The relationship was between God and the natural man of the nation of Israel. They lived exclusively in the flesh as they tried to relate to God. 

The circumcision of the New Covenant is inward and manifests with the Holy Spirit residing in the heart of the believer. This is made possible when one is reborn from above. The spirit of that person is made alive to the Spirit of God. We refer to this reality as being one with Christ. We are still in the flesh, but relate to Him in Spirit. This is true worship, for God is Spirit 

In these verses we see the fullness of what it means to be a Jew. It is the one in relationship with God. The fulfillment of that relationship is in a people who approach God the Father as His children.