Colossians 2:11 says, "In Him also you were circumcised with a circumcision not made with hands, in the putting off of the body of the flesh, in the circumcision of Christ."
Philippians 3:3 says, "For we are the circumcision, the ones who serve by the Spirit of God and boast in Christ Jesus and have no confidence in the flesh."
These two verses show what circumcision is. Simply put, circumcision is the removal of the flesh. What should be the attitude of those who are circumcised? They should have no confidence in the flesh and should not put their trust in the flesh. (The word "confidence" in Philippians 3:3 can be translated "trust" according to the original language.) Who are the circumcision? They are the ones who serve by the Spirit of God and who put no trust in the flesh. Therefore, circumcision deals with man's inherent energy, his natural strength.
How appropriate it was for God to say such a word to Abraham! God showed Abraham that whatever he did and begot by himself was just Ishmael. If the flesh is not dealt with, one will have no part in God's covenant. If the flesh is not dealt with, one cannot be of God's people and cannot maintain His testimony or participate in His recovery work.
The greatest problem among God's children is that they do not know what the flesh is! The flesh that many Christians know is merely related to the matter of sin. It is true that the flesh causes us to sin. But the flesh does not cause man to just sin. Romans 8:8 says that "those who are in the flesh cannot please God." This means that the flesh has tried to please God. Many times, the goal of the flesh may not be to try to offend God; its goal may be to try to please God. Romans 7 shows us that the flesh exerts great effort to keep the law, to do good, to do God's will, and to please God. However, it cannot make it. Our experience tells us that it is comparatively easy to deal with the sinning flesh, but it is very difficult to deal with the flesh that tries to please God. This is the flesh that tries to creep into God's work and service. This is the flesh that creeps into all the things of God.
Some people do not realize that man cannot please God by himself. They think that even though they were once a certain kind of person, now they can do good because their goal has changed since they have believed in the Lord. Such people do not realize that God is concerned not merely with changing their goals, but with terminating their flesh. If they want to please God with their flesh, God will tell them that the flesh cannot please Him. We must see that circumcision is the cutting off of the flesh, the very flesh that begets Ishmael, the flesh that tries to please God. Circumcision deals with the flesh that tries to do God's will and to fulfill His promise by itself. This was what God wanted Abraham to understand.