Why do believers lose the fullness of the Holy Spirit over time?
At salvation, believers are filled to the brim with the Holy Spirit, but God leaves it within their control how much fullness they maintain. As they pursue the world's allurements, temptations, fleshly desires, and material things, these displace the Spirit's fullness, so their cup empties over time.
Les teaches that the moment a person is saved, God puts into that person all of Himself that He can, and the believer has the fullness of the Holy Spirit from the very moment of belief. It is not something one has to work for or look for; it was all part of salvation. However, God leaves within the believer's realm of control how much of that fullness is maintained or allowed to empty out. Using a cup analogy, Les compares the Holy Spirit to air, or pneuma. Just as pouring water out lets air fill the cup, believers can start filling their cup with material things. Because they remain in this old world with its allurements, temptations, and desires of the flesh, those things can displace the Spirit's fullness until the cup is full of the material instead.