Circumcision
Technically, circumcision is the removal of the male foreskin. Today the procedure is commonly performed on newborn boys, mainly for health reasons. In ancient times it was practiced among certain people groups. When God made a covenant with Abraham, the founder of the Jewish nation, he asked Abraham to circumcise all the males in his family and his servants' families as a sign of the promise God had arranged with him. This set Abraham's descendants apart from the people around them, reminding them that they must follow God's laws. While other groups practiced circumcision, it was not for reasons of a covenant with God.

With Jesus Christ's death and resurrection in the New Testament, circumcision is no longer needed to prove faithfulness to God. So when the New Testament mentions circumcision, it is often used to represent the entire Mosaic Law, and it refers to people who try to win approval from God by simply doing the "right" things could be going to church, reading the Bible and praying every day. None of these things prove that someone is a follower of Jesus Christ. Instead of circumcising our bodies or doing all the "right" things, God tells us to circumcise our hearts, which means that our hearts must be changed as we follow and obey God.

Old Testament circumcision was mainly a Jewish practice to illustrate a man's willingness to obey God and God's promise to his people.
Genesis 17:10-14
Exodus 12:48
Deuteronomy 10:16-17
Joshua 5:2-9
The New Testament calls for an inward circumcision of the heart, stating that outward circumcision is no longer the required sign for acceptance of God's covenant.
Romans 2:25-29
1 Corinthians 7:18-19
Galatians 3:28-29; 5:2-6