Why does the believer have a spiritual battle at all?
The believer has a spiritual battle because the indwelling Holy Spirit creates the battle line against the still-present old Adam. Without the Holy Spirit, there would be no warfare, and the believer would not stand a chance against the forces of old Adam and Satan.
Les explains that old Adam was crucified and in God's eyes is completely dead, out of the picture, but in our experience he is still here, though defeated. He doesn't have to remain uppermost; if he does, it's because we're not fighting the battle. The Holy Spirit came in and empowered our spirit (body, soul, and spirit) so that, working in consort with our spirit, we can live victoriously. But old Adam is powerful and will come to the fore at times. This is where the warfare exists: there would be no warfare were it not for the indwelling Holy Spirit, otherwise there wouldn't even be a battle line. We wouldn't stand a chance against the forces of old Adam and Satan if we didn't have the Holy Spirit. Les also notes the Greek Pneuma Hagion, meaning power from on high.