Why do believers need the Pneuma Hagion to overcome the old nature?
Believers need the Pneuma Hagion because it is the power from on high exercised through the Holy Spirit. By that power they can confront the old nature and live victoriously in the flesh. Without it they cannot overcome; by faith they draw on the Spirit rather than forcing themselves to be law-keepers, letting Him run the race.
Les explains that the Holy Spirit is the person, while the Pneuma Hagion is the power from on high that is exercised through the person of the Holy Spirit. This is the power believers need to confront the old nature and to live victoriously in the flesh, and they cannot do it without the Pneuma Hagion. Les says this must be understood and received by faith: the believer can say, Lord, I have the Holy Spirit, I can draw on Him, and I do not have to force myself to be a law keeper. It then comes naturally to let the Spirit run the race for us. He also contrasts those after the flesh, who mind the things of the flesh and know nothing of God's saving grace.