The people of Israel were required by God to have a sign in their flesh that would remind them of their commitment to the covenant they entered into with God. Just as a wedding ring is an outward sign that a person is in a marriage covenant with another, so a Hebrew man had a sign on his flesh that was forever with him to remind him of his commitment to the God of Abraham, Isaac, and Jacob. Here's God's instructions to Abraham:
9Then God said to Abraham, “As for you, you must keep my covenant, you and your descendants after you for the generations to come. 10This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised. 11You are to undergo circumcision, and it will be the sign of the covenant between you and me. 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 (Genesis 17:9-13).
Commentator R. Kent Hughes says this of the mark of circumcision:
“Significantly, circumcision involved Abraham’s powers of procreation—the area of life in which he had resorted to fleshly expediency and had so failed. Man’s best plans and strength of will would never bring about the promise. For Abraham circumcision was an act of repentance and a sign of dependence upon God for the promise.”
Abraham tried to bring about God's will and purpose through doing what he could do (as in the matter of Sarai's servant girl, Hagar, giving birth to Ishmael), and had miserably failed. God was now showing that He alone could satisfy their every need and fulfill the vision that He had given them. Abraham tried to bring about the vision by their means, but now it was time to look to the Lord and do things His way. He was finally at rest and waiting for God to fulfill His Word and His promises, by doing things in God’s way and in His timing. As believers in Christ, this is a lesson we also must learn—to do things in God’s way, not ours.
Later on, in the New Testament, circumcision became a matter of considerable controversy among believers in the early church. When non-Jews became believers, some argued that they should be circumcised as well. Paul explained that this was no longer necessary. His argument followed the reasoning that believers should be circumcised at the heart level, and not focus on that which is physical.
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 2:28-29).
Resting in what God has done for us and on our behalf is the Sabbath rest of faith. No more do we have to please God by signs on the outer skin of our commitment to keeping the way of the Lord. Now it is a matter of the inward heart. The Spirit leads and guides us from within, not by a written code of trying to please God by the works of the flesh but realizing that God has come down from heaven and satisfied all the demands of the Law. We wish to please God from the heart, not from the flesh.
Thank God that we now have a new covenant, sealed with Messiah's blood. He has given us what we need to be perfect and complete. When Christ died for our sanctification [set apart for holy use], circumcision was no longer required, as He is our sanctification (1 Corinthians 1:30). Paul explained it well when he wrote: "For it is we who are the circumcision, we who worship by the Spirit of God, who glory in Jesus Christ, and who put no confidence in the flesh" (Philippians 3:3). Keith Thomas
 R. Kent Hughes, Genesis, Beginning and Blessing. Published by Crossway, Page 248.