Col 2:11-12, Gal 6:12-15, Rom 2:29
In the Old Testament, God required people to be circumcised in order to become His people. (Genesis 17:9-13)
Genesis 17:9 And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations. 10 This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised. 11 And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you. 12 And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. 13 He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant.
The physical circumcision that God required in the Old Testament was to explain spiritual circumcision. We are spiritually circumcised by believing that Jesus died for us and that God raised him up again. (Col 2:11-12, Gal 6:12-15, Romans 2:29)
Col 2:11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: 12 Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead.
Gal 6:12 As many as desire to make a fair shew in the flesh, they constrain you to be circumcised; only lest they should suffer persecution for the cross of Christ. 13 For neither they themselves who are circumcised keep the law; but desire to have you circumcised, that they may glory in your flesh. 14 But God forbid that I should glory, save in the cross of our Lord Jesus Christ, by whom the world is crucified unto me, and I unto the world. 15 For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature.
Romans 2:29 But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.