A selection of verses relating to circumcision.
It is the token of the covenant between God and the Jews:
11 You shall be circumcised in the flesh of your foreskin. It will be a token of the covenant between me and you.
The letter to the Romans tells us that there is more to circumcision than just the physical sign:
28 For he is not a Jew who is one outwardly, neither is that circumcision 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 praise is not from men, but from God.
The same letter points out that Abraham is approved by God because of his faith before he is circumcised:
11 He received the sign of circumcision, a seal of the righteousness of the faith which he had while he was in uncircumcision, that he might be the father of all those who believe, though they might be in uncircumcision, that righteousness might also be accounted to them.
The letter to the Galatians tells us that we can become heirs to the promises made to Abraham by having faith in Christ and being baptised:
26 For you are all children of God, through faith in Christ Jesus. 27 For as many of you as were baptised into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus. 29 If you are Christ’s, then you are Abraham’s offspring and heirs according to promise.