Why is the Christian life a royal battle between the old and new natures?
The Christian life is a royal battle because God reckons the old Adam crucified and dead, yet in experience that old nature remains with us. At regeneration God creates a new divine nature in the spirit, so the two natures work in consort upon the physical body, producing ongoing conflict.
Les explains that the old Adam has been crucified and God reckons him as dead, but in experience the old nature is still with us. At regeneration God immediately creates a new nature, a divine nature, opposite to the old. With the regenerated spirit, the two work in consort and influence the physical body. That leaves the believer in a royal battle: the old nature is reckoned dead yet still present. Les compares this to Christ defeating Satan, who is defeated yet still alive and well.