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. 8:8). But God condemned sin in the flesh at the cross (Rom. 8:3).
When a sinner believes on the Lord Jesus Christ, he is born of God, receiving a new nature which cannot sin (1 John 3:9). But the old nature remains in him still, and not one bit improved by the presence of the new. Thus an unconverted man has one nature which is sinful; a Christian has two natures: the old, sinful one and the new one which cannot sin.
Many, when they are saved, are surprised and downcast 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 Rom. 6:6,7 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 on 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 verse 11 to reckon ourselves "to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.”
It does not say to realize, nor feel, nor experience, but to reckon. 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." John 3:6. The Young Christian
I 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