Why is there a spiritual battle in believers at all?
The spiritual battle exists because the indwelling Holy Spirit creates a battle line against old Adam, who, though crucified and defeated in God's eyes, is still present in the believer's experience. Without the Spirit there would be no warfare, and the believer would stand no chance against old Adam and Satan's forces.
Les explains that once old Adam is crucified, in God's eyes he is completely dead and out of the picture, yet in the believer's experience he is still present, though defeated. Old Adam does not have to remain uppermost; if he does, it is because the believer is not fighting the battle. The Holy Spirit came in and empowered the believer's spirit, working in consort with it so the believer can live victoriously. Old Adam is powerful and will come to the fore at times, and that is where the warfare exists. There would be no warfare at all were it not for the indwelling Holy Spirit; otherwise there would be no battle line, and the believer would not stand a chance against the forces of old Adam and Satan. That is the secret: the Holy Spirit, the Pneuma Hagion, is the power from on high.