What does it mean that the old sin nature is declared dead?
It means God reckons the old Adam, or sin nature, as crucified and dead, so that in His sight it is as if the person never sinned. This declaration does not remove the sin nature from daily experience; it remains a viable force, requiring God's intervention for control and defeat.
Les teaches that the cross brings Paul's explicit teaching about the sin nature: this sin nature can be crucified and literally by faith committed unto God, to have Him put it to death. So the old sin nature has to be declared dead, at least so far as God is concerned, although it remains a viable force in our living. The minute God reckons old Adam as crucified, He puts within us a new nature, a divine nature, which is the very mind and nature of Christ Himself; that is what it means to become Christlike, or Christian. In justification, God reckons old Adam as dead and sees him no more, just as if we had never sinned, but in the believer's experience he is still there. The only hope is that God has intervened with something that will help us control this old Adam and keep him defeated.