The believer who would be truly spiritual must recognize the presence of the old nature within. It would be dangerous not to recognize a foe so near.
The old nature in the believer is that which is “begotten of the flesh.” It is called, “the flesh,” “the old man,” “the natural man,” “the carnal mind.”
Just as “they that are in the flesh cannot please God” (Rom. 8:8) so that which is of the flesh, in the believer, cannot please God. “The flesh,” as we have already seen, is totally depraved. God calls it “sinful flesh” (Rom. 8:3), warns that it seeks “occasion” to do wrong (Gal. 5:13), and declares that “the works of the flesh” are all bad (Gal. 5:19-21).
Nor is the old nature in the believer one which improves by its contact with the new. It is with respect to “the flesh” in the believer, even in himself that the Apostle declares that in it “dwelleth no good thing” (Rom. 7:18), that it is “carnal, sold under sin” (Rom. 7:14), that it is “corrupt according to the deceitful lusts” (Eph. 4:22), that it is at “enmity against God,” and is “not subject to the law of God, neither indeed can be” (Rom. 8:7).
“The flesh,” even as it remains in the believer after salvation, is that which was generated by a fallen begetter. It is the old Adamic nature. It is sinful in itself. It cannot be improved. It cannot be changed. “That which is born [begotten] of the flesh is flesh,” said our Lord (John 3:6), and it is as impossible to improve the “old man” in the believer as it was to make him acceptable to God in the first place.
The “old man” was condemned and dealt with judicially at the Cross. Never once is the believer instructed to try to do anything with him or to make anything of him, but always to “reckon” him “dead indeed” (Rom. 6:11), and to “put him off” (Col. 3:8-10). source