Man is a fallen creature, born in sin, with an evil nature that does nothing but sin against God. This evil nature is utterly incorrigible, and cannot please God. (Rom. 8:88So then they that are in the flesh cannot please God. (Romans 8:8).) But God condemned sin in the flesh at the cross. (Rom. 8:33For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: (Romans 8:3).) And when a sinner believes on the Lord Jesus Christ, he is born of God, receiving a new nature which cannot sin. (1 John 3:99Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. (1 John 3:9).) But the old nature remains in him still, and not one whit improved by the presence of the new. Thus An unconverted man has one nature, sinful; A Christian has two, the old sinful one, and the new which cannot sin.
Many, when they are saved, are surprised and cast down because they sin again, and they sometimes even fear that they are lost after all. This arises from the erroneous thought that their old evil nature is improved, and thus Satan gets an advantage.
In Romans 6:66Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. (Romans 6:6), we read, “Knowing this, that our old man is crucified with Him, that the body of sin might be destroyed (not improved), that henceforth we should not serve sin. For he that is dead is freed (justified) from sin.” God has given up the old man; it was crucified once for all upon the cross. Believe God, and you are freed from sin. It is no longer your master.
Then, as to the practical side, we are taught in the 11th verse to reckon ourselves “to be dead indeed unto sin, but alive unto God through (in) Jesus Christ our Lord.”
Not to realize, nor feel, nor experience, but to reckon. And there would be no need for such an exhortation if sin were not still in us, or if the old nature were made better.
“That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.”