Ephesians 2:11 Therefore remember that once you, the Gentiles in the flesh, those who are called uncircumcision by that which is called circumcision in the flesh made by hands, (12) That you were at that time apart from Christ...
The man [Adam] God created to fulfill His purpose was pure, without sin or any kind of negative mixture. However, sin, the evil nature of Satan, entered into man through the fall. When the evil nature of Satan entered into man, it firstly caused man's body to become the flesh, full of lusts, and eventually caused the whole being of man to become the flesh.
The body was the pure vessel created by God; the flesh is the corrupted body. God did not create the lusts in man's body... Fallen people live according to the flesh, not according to the spirit, the conscience, or reason....
Because his whole being had become the flesh, man was damaged and thus prevented from fulfilling God's purpose. Since mankind as a whole could not fulfill His purpose, God came in to call out of fallen mankind a race-Abraham and his descendants-for the accomplishment of His purpose. God then commanded Abraham and his descendants to be circumcised, that is, to put away the flesh.
Thus, circumcision indicates that God's chosen people must put aside their flesh. The fact that the called race was circumcised meant that they were separated from fallen mankind and delivered out of the fallen condition. Circumcision made a tremendous distinction between them and the rest of mankind.
[That was in the Old Testament. Today, according to the New Testament in Romans 2:28, “For he is not a Jew who is one outwardly; neither is circumcision that which is outward in the flesh. (29) But he is a Jew who is one inwardly; and circumcision is of the heart, in spirit, not in letter, whose praise is not from men, but from God.”]