When you came to Christ, you were “circumcised,” but not by a physical procedure. Christ performed a spiritual circumcision—the cutting away of your sinful nature.
As stated in this verse, circumcision means a cutting away, and in this reference it refers to a cutting away of the old sinful nature that used to reside on the inside. When you truly become born again, the real you on the inside died and a new spirit was birthed. The old has passed away and the new has come. So many who claim to be Christians still live like the world. How can this be since you have been recreated in the image of God inside?
One answer is we have many false converts. People who think they are born again but in fact they are not. They would say, “I prayed the prayer” or “I went to church when I was a child”. You see 1 John 3:7-8 tells us when you are born again you no longer have a desire to practice sin. In other words, you do not want to live contrary to Christ. A baby Christian, as they continue their walk with God, will begin to realize from within to live in sin is wrong. As they grow they will no longer desire to do so.
When you come to Jesus with the knowledge of your sinful nature within you repent, turn away from that old nature, and make Jesus the Lord of your life. If you are born again that old nature has been cut away and a new nature, the nature of Christ has come. As you get to know Him, and renew your mind to this new man on the inside, that new nature will change who you are on the outside.