Romans 2:29

No, a man is a Jew if he is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a man's praise is not from men, but from God.

(v29, NIV)

Being a Jew means being a Jew inwardly. You can't call yourself a Jew if you're simply one outwardly. Circumcision is not merely outward either. Instead, inwardly, your heart is circumcised. This inward circumcision is not by the written code. The Jews were circumcised and followed the written law. However, I guess these hypocritical Jews weren't circumcised of the heart, because they were not circumcised by the Spirit, but by the written code. This Spirit is what the Jews didn't get or didn't understand. They lived by the law and while living by the law, they didn't live by it in its entirety. They couldn't and they didn't have the Spirit. The Spirit is that which gives life and if you don't have the Spirit but you have the law, you're still lost.