Why did God command Abraham to cut off the foreskin?
Many of the commands that God gave man early on appeared to be meaningless at the time it was given. Only later did God reveal the meaning of the practice.
As an example, God commanded that the Israelites offer up incense during the worship. "Aaron shall burn on it sweet incense every morning; when he tends the lamps, he shall burn incense on it. And when Aaron lights the lamps at twilight, he shall burn incense on it, a perpetual incense before the LORD throughout your generations" (Exodus 30:7-8). There were very strict rules about what incense could be burnt and who was allowed to burn these incenses (only the priests were allowed). Not until hundreds of years later did God mention that incense represented prayers to God. "Let my prayer be set before You as incense, the lifting up of my hands as the evening sacrifice" (Psalm 141:2).
Abram, as part of his covenant with God, was commanded to circumcise his foreskin. "This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised; and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. He who is eight days old among you shall be circumcised, every male child in your generations, he who is born in your house or bought with money from any foreigner who is not your descendant. He who is born in your house and he who is bought with your money must be circumcised, and My covenant shall be in your flesh for an everlasting covenant. And the uncircumcised male child, who is not circumcised in the flesh of his foreskin, that person shall be cut off from his people; he has broken My covenant" (Genesis 17:10-14).
The immediate reason was that every covenant contained a clause concerning the witness to the covenant. A witness was a perpetual reminder that the covenant existed. For example, the covenant between Jacob and Laban was witnessed by a stone marker (Genesis 31:46-48). God's covenant with Noah and the world was witnessed by the rainbow (Genesis 9:12-16). In Abram's case, God asked that he and all his male descendants who entered into the covenant carry the reminder of the covenant in their own flesh.
Centuries later, God revealed what circumcision represented. "But if they confess their iniquity and the iniquity of their fathers, with their unfaithfulness in which they were unfaithful to Me, and that they also have walked contrary to Me, and that I also have walked contrary to them and have brought them into the land of their enemies; if their uncircumcised hearts are humbled, and they accept their guilt- then I will remember My covenant with Jacob, and My covenant with Isaac and My covenant with Abraham I will remember; I will remember the land" (Leviticus 26:40-42). Hence, uncircumcision represented stubborn sinfulness. Circumcision was done on the outward flesh, but it represented the acceptance of the covenant in the mind, including the willingness to obey the laws within the covenant. When God told Israel, "Therefore circumcise the foreskin of your heart, and be stiff-necked no longer" (Deuteronomy 10:16), it meant that they were remove their stubborn sinful thoughts from their minds. In other words, they were to purge sin from their lives and become obedient to the laws of God. "And the LORD your God will circumcise your heart and the heart of your descendants, to love the LORD your God with all your heart and with all your soul, that you may live" (Deuteronomy 30:6).
While circumcision served as a permanent reminder to Israel that they needed to obey God's laws, the fact that a man was circumcised did not mean he actually kept the law. "For circumcision is indeed profitable if you keep the law; but if you are a breaker of the law, your circumcision has become uncircumcision" (Romans 2:25). Hence, to God the physical act of circumcision was not nearly as important as the actual obedience that He required. "Therefore, if an uncircumcised man keeps the righteous requirements of the law, will not his uncircumcision be counted as circumcision? And will not the physically uncircumcised, if he fulfills the law, judge you who, even with your written code and circumcision, are a transgressor of the law? For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; 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" (Romans 2:26-30).
Circumcising the foreskin is not a part of God's covenant with Christians. "Was anyone called while circumcised? Let him not become uncircumcised. Was anyone called while uncircumcised? Let him not be circumcised. Circumcision is nothing and uncircumcision is nothing, but keeping the commandments of God is what matters" (I Corinthians 7:18-19). However, Christians still have an act that represents circumcision -- a witness to the acceptance of God's covenant. Since the emphasis is placed on the removal of sin from a person's life, this equivalent act to circumcision represents the removal of sin. "In Him you were also circumcised with the circumcision made without hands, by putting off the body of the sins of the flesh, by the circumcision of Christ, buried with Him in baptism, in which you also were raised with Him through faith in the working of God, who raised Him from the dead" (Colossians 2:11-12). Hence, baptism represents the removal of sin from a person's life. "'And now why are you waiting? Arise and be baptized, and wash away your sins, calling on the name of the Lord" (Acts 22:16). Or as Paul later wrote: "Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin" (Romans 6:3-7).
It is sad to hear people claim that baptism is unnecessary. It would be equivalent to an Israelite claiming that circumcision was unnecessary. Refusal to accept baptism is a refusal to accept the covenant of Christ, just as an uncircumcised descendant of Abraham was a covenant breaker. You cannot claim to be under the covenant of Christ unless you are willing to take on the witness to that covenant.