The believer who would be truly spiritual must recognize the presence of the old Sin-nature is still within him. It would be dangerous not to recognize a foe so near as “Sin in the flesh” (Romans 8:3).

The old nature in the believer is that which is “begotten of the flesh.” It is called, “the flesh” (Rom 8:1), “the old man” (Rom 6:6; Eph 4:22; Col 3:9), “the natural man” (1Cor 2:14), “the carnal mind” (Rom 8:7).

Just as “they that are in the flesh cannot please God” (Rom. 8:8), the works which are of the flesh, in the believer, cannot please God. The flesh includes “the mind of the flesh” which “is at enmity with God” (Rom 8:7). “The flesh,” as we have already seen, is totally depraved at its core. God through Paul calls it “sinful flesh” (Rom. 8:3), warning us that the flesh seeks “occasion” to do wrong, and declares that “the works of the flesh” are all bad (Gal. 5:19-21).

“For, brethren, ye have been called unto liberty (freedom); only use not liberty for an occasion (an opportunity) to the flesh, but by love serve one another.” (Galatians 5:13)

The old nature in the believer is one which will never improve even in light of the new Christ nature in believers. Paul plainly declares that “in me, (that is in my flesh) dwelleth no good thing” (Rom. 7:18 KJV exact). Paul says he as a believer is still inherently “carnal, sold under sin” (Rom. 7:14).

Elsewhere, Paul declares that the old (Sin) nature is “corrupt according to the deceitful (lying) lusts” (Eph. 4:22), which it is “at enmity against God,” and is “not subject to the law of God, NEITHER INDEED CAN BE” (cf. Rom. 8:7-8).

The nature of “the flesh” remains in the believer after salvation. It is the fallen nature of Adam that was generated by an evil begetter, the Serpent. The “Devil” (Gk. diabolos) is the usurping false father of all humanity while they live in human flesh. Thus of the Pharisees, Jesus said “ye (you all) are of your father the devil”; and of Peter who wanted to prevent Jesus from dying, Jesus said “get thee behind me Satan” (Mark 16:33). The “flesh” of the “old man” is inherently sinful in itself. It cannot be improved. It cannot be changed. “That which is born [begotten] of the flesh is flesh,” said our Lord Jesus (John 3:6) and it is as impossible to improve the “old man” and it cannot make any man acceptable to God.

The title “Devil,” is the Greek “diabolos” taken from “diaballo.” The prefix “dia” means “to divide” and “ballo” a “ball.”  It’s as if the Devil’s work is to roll accusations against believers and roll ideas into the fallen mindset of the flesh in order to “divide” men from the Lord and His truth.

Thankfully the “old man” of “the flesh” was judged and condemned at the cross.

Our “old man” was crucified with the Lord.

“Know ye not, that so many of us as were baptized (repositioned) into Jesus Christ were baptized (repositioned) into his death? (Rom. 6:3)
“I (as to my “old man”) am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.” (Gal. 2:20)

So then, what is left for us to do since we still live in sinful flesh bodies.

Never once is the believer instructed to make anything of the flesh, except to always “reckon (count)” him dead, and so to “put (him) off” the “old man” of “the flesh.”

“Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.” (Romans 6:11 (KJV)

“Lie not one to another, seeing that ye have put off the old man with his deeds;” (Colossians 3:9)