Romans 2
25 For circumcision is indeed profitable if you keep the law; but if you are a breaker of the law, your circumcision has become uncircumcision. 26 Therefore, if an uncircumcised man keeps the righteous requirements of the law, will not his uncircumcision be counted as circumcision? 27 And will not the physically uncircumcised, if he fulfills the law, judge you who, even with your [f]written code and circumcision, are a transgressor of the law? 28 For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; 29 but he is a Jew who is one inwardly; and circumcision is that of the heart, in the Spirit, not in the letter; whose [g]praise is not from men but from God.

The Jew believed that his circumcision guaranteed his salvation. He might be punished in the world to come, but could never be lost.

Circumcision and baptism do about the same thing that a label on a can does. If the outer label doesn’t match with what is on the inside, something is wrong! If there are carrots inside the can, you can put a label that says “Peas” but it doesn’t change what is inside the can. Being born again changes what is inside the can, and then you can put the appropriate label on the outside.