There are several reasons why Christians reject circumcision. As stated above circumcision was a sort of sign or mark of the covenant between man and God. Although the bible states that this covenant was to be everlasting the authors of the New Testament decided to extend the promises of God that at one time were believed to be exclusively for the Jews to gentiles as well. This provision was already made in the first covenant when it stated:
12For the generations to come every male among you who is eight days old must be circumcised, including those born in your household or bought with money from a foreigner—those who are not your offspring. 13Whether born in your household or bought with your money, they must be circumcised. My covenant in your flesh is to be an everlasting covenant. 14Any uncircumcised male, who has not been circumcised in the flesh, will be cut off from his people; he has broken my covenant.” Genesis 17:12-14
As you can see in the above cited text if you resided with the Jews even as a foreigner who was bought as in a servant etc. you had to be circumcised! By being circumcised you are made an heir to the promises of the covenant. Initially the New Testament states that Jesus only came to redeem the lost sheep of Israel but that later salvation was made available to all. We will get to that in a moment.
First let's put the Bible in perspective by defining the term covenant and how this is relevant to the old and new testaments.
covenant: a usually formal, solemn, and binding agreement
testament: archaic : a covenant between God and the human race
As seen above according to the Merriam Webster dictionary the words covenant and testament are interchangeable because both words in essence have the same meaning. So knowing this when you think of the bible, think of the Old Testament in terms of the old covenant and the new in terms of the new covenant. It simplifies things greatly and makes my job easier to explain these concepts to the layman more clearly.
One of the earliest disagreements that the early church faced was on whether or not gentile converts were required to be circumcised as prescribed under the law. In the book of Matthew Jesus is approached by a Canaanite woman who pleads with him to liberate her daughter from demon possession. Jesus did not immediately respond but when prompted by his disciples he replied: “I was sent only to the lost sheep of Israel.” Matthew 15:24 when the lady pleaded further he made this disgusting and disparaging remark: “It is not right to take the children’s bread and toss it to the dogs.” Matthew 15:26 she came back with a witty reply and Jesus was so impressed by her faith that he granted her request.
The Christians did what they often do. They quote texts out of contexts in order to reinterpret those texts to validate their current new view of their own religion. In the New Testament the goal is to make the promises of Israel and Judah extend to the rest of the world by inventing a new covenant with Christ as the mediator. So the first thing Christ is made to say is that he is not here to abolish the law or the prophets but to fulfill it. Matthew 5:17 This makes Christ the mediator of the new covenant made available to all.
In Hebrews chapter eight this so called declaration of Christ is contradicted by Paul when he states that: 7For if there had been nothing wrong with that first covenant, no place would have been sought for another. 13By calling this covenant “new,” he has made the first one obsolete; and what is obsolete and outdated will soon disappear. Hebrews 8:7,13 Of course the author of Hebrews between verses 8-12 cites a promise from Jeremiah 31:31-34 specifically referring to the Israel and Judah as being a promise of a new covenant between them and God. Christians do this often in the New Testament. They claim promises exclusive for the people of Israel as their own by citing those same promises out of context.
From that base of reinterpreting Old Testament texts Christians begin to build their new religion. I have always stated that all religions borrow and build upon one another and this is made most apparent with a careful reading and cross referencing of the Tanakh, the New Testament, and the Koran. With this new interpretation of Old Testament promises those made to Abraham and his heirs is now extended to the gentiles.
He (Jesus) redeemed us in order that the blessing given to Abraham might come to the Gentiles through Christ Jesus, so that by faith we might receive the promise of the Spirit. Galatians 3:14
Here the Christians claim the blessings of Abraham with the omission of the mandatory circumcision that was the main stipulation that God had set as mans part of the bargain.
10...Every male among you shall be circumcised. 11You are to undergo circumcision, and it will be the sign of the covenant between me and you. 12For the generations to come every male among you who is eight days old must be circumcised, including those born in your household or bought with money from a foreigner—those who are not your offspring. Genesis 17:10-12
Notice that the promise is available to foreigners but that circumcision is mandatory. So how did the early church get around this little stipulation? They invent a new covenant through the invention of a new mediator. This new mediator breaks the stipulations of the old covenant by doing away with it entirely and making circumcision not a physical act but rather a spiritual one.
25Circumcision has value if you observe the law, but if you break the law, you have become as though you had not been circumcised. 26So then, if those who are not circumcised keep the law’s requirements, will they not be regarded as though they were circumcised? 27The one who is not circumcised physically and yet obeys the law will condemn you who, even though you have the written code and circumcision, are a lawbreaker.28A person is not a Jew who is one only outwardly, nor is circumcision merely outward and physical. 29No, a person is a Jew who is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a person’s praise is not from other people, but from God. Romans 2:25-29
In him you were also circumcised with a circumcision not performed by human hands. Your whole self ruled by the flesh was put off when you were circumcised by Christ, Colossians 2:11
6This mystery is that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus. Ephesians 3:6
Note: All biblical citations are from the New International Version and all word definitions come from the Merriam Webster dictionary.