How can the sin nature be crucified?

By faith, the sin nature is committed to God so He puts it to death; God reckons old Adam as crucified, declares it dead, and replaces it with a divine nature—the mind of Christ. Though still present experientially, it is defeated through God's intervention.

Les teaches that Paul was the first to explain the sin nature and how it can be crucified: by faith it is committed to God so that He puts it to death. God reckons old Adam as crucified and declares the sin nature dead, at least as far as God is concerned, even though it remains a viable force in our living. The moment God reckons old Adam crucified, He puts within us a new nature—the very mind and nature of Christ—which is what it means to become Christlike or Christian. Justification means it is just as if we never sinned, as if old Adam is completely out of God’s sight. But in the believer’s experience, old Adam is still there. The only hope is that God has intervened with something to help us control this old Adam and keep him defeated.

Who first taught in the Bible that the sin nature can be crucified?

The Apostle Paul first taught explicitly in Scripture that the sin nature can be crucified. This crucifixion is appropriated by faith, committing the old sin nature to God so He puts it to death, and it must be declared dead so far as God is concerned.