Circumcision is the act of cutting off the foreskin of a penis. Male circumcision is still practiced by Jews (a practice called Brit milah, which usally takes place when a baby boy is 8 days old) and Muslims today, although most Christians do not practice circumcision.
Circumcision in the Old Testament
"God said to Abraham, “As for you, you will keep my covenant, you and your seed after you throughout their generations. This is my covenant, which you shall keep, between me and you and your seed after you. Every male among you shall be circumcised. You shall be circumcised in the flesh of your foreskin. It will be a token of the covenant between me and you. He who is eight days old will be circumcised among you, every male throughout your generations, he who is born in the house, or bought with money from any foreigner who is not of your seed. He who is born in your house, and he who is bought with your money, must be circumcised. My covenant will be in your flesh for an everlasting covenant. The uncircumcised male who is not circumcised in the flesh of his foreskin, that soul shall be cut off from his people. He has broken my covenant.”(Genesis 17:9-14 WEB).
This commandment was reinforced in the covenant of Moses (for example, in Leviticus 12:3). However, Deuteronomy 10:16 states, "Circumcise therefore the foreskin of your heart, and be no more stiff-necked."
Circumcision in the New Testament
Jesus was circumcised (Luke 2:21), and presumably his first disciples were as well. However, a controversy raged in the early church: was circumcision only for the Jews, or should Gentile converts to Christianity also be circumcised? The controversy is first mentioned in Acts 15 (the Council of Jerusalem). A group of Pharisee Christians, commonly known as Judaizers, argued that Gentile converts to Christianity were obligated to be circumcised. Paul, on the other hand, argued that Gentiles were under the covenant of Christ, and that the covenants of Moses and Abraham did not apply to Gentiles. James the Just ruled in favor of Paul. Paul wrote his epistle to the Galatians shortly after, to outline his position on circumcision.
For Paul, the true circumcision was of the heart, and not of the flesh (Romans 2:28-29); thus "circumcision is nothing, and uncircumcision is nothing (1 Corinthians 7:18-19). Paul was not opposed to circumcision among Jewish Christians. He circumcised Timothy, who had a Jewish mother and was thus considered to be a Jew. Rather, he was opposed to the false circumcision (Philippians 3:2-3), those who hypocritically elevate a mark in the flesh above acceptance of the Spirit of God.
Circumcision in Christianity
The Roman Catholic Church formally condemned circumcision in 1442 at the Council of Florence, and most Protestant churches have followed suit. Circumcision is neither required or condemned by the Eastern Orthodoxy, although some Oriental Orthodox churches (such as the Coptic Orthodox Church of Alexandria) still practice circumcision.