What does it mean that the body of sin is destroyed?
That the body of sin is destroyed means the old Adam has not been removed or annihilated but has been knocked out of commission. It has lost its power, so henceforth believers should not serve old Adam, though the power can be reengaged if one goes back and flips it on.
Les explains that the body of sin refers to the old Adam, and "destroyed" in the Greek does not mean removed and annihilated; it means knocked out of commission. The old Adam has lost its power. Les illustrates this with a main fuse box: you can pull the lever and all the lights go out, but the power is still there, and somebody can go back and flip it up so the lights come back on. He says that is what has happened to the old Adam—he has lost his power—so henceforth we should not serve old Adam. This connects with identification by faith: when Christ was on the cross, He died your death, God saw you on the cross in the person of Christ, in the tomb those three days, and in resurrected life. Knowing this is only because the Word says so; you cannot feel it or experience it except by faith that our old Adam is crucified with Him. The unbeliever, by contrast, is living to satisfy the desires of old Adam.