How can believers control the activity of the body?
Believers can control the activity of the body by keeping the spirit part of themselves exercised through the Word, prayer, and Christian fellowship. When the spirit withdraws from the battle line, old Adam seizes the opportunity and controls the body's acts, leading to sin.
Les explains that we all fail and fall, and when the spirit part withdraws from the battle line, old Adam jumps at the opportunity and begins to control the acts of the body, causing believers to sin. But the whole idea is to keep that spirit part literally exercised by the Word, by prayer, by Christian fellowship, and then we can pretty much control the activity of the body. Even though the two natures are contrary, if we are led of the Spirit, we are not under the Law. Paul's dilemma in Romans 7 gives comfort that we are not alone.