Can a believer choose to be Spirit-filled instead of fleshly-minded and still be a believer?
Yes. Scripture implies a believer can choose to be Spirit-filled rather than fleshly-minded, and both are still believers. Such a choice matters because God disciplines fleshly believers; He does not chasten unbelievers. A believer has free exercise of will, but sin can bring chastening, even the sin unto death.
Les explains that the implications from Scripture are to choose to be a Spirit-filled believer and not a fleshly-minded believer, yet both are still believers. He warns that a believer can let old Adam reign in the mortal body and obey its lusts; it can happen, but God is not pleased. When people say such a person is no longer saved, Les says no. If a true believer fools around in sin and brings reproach on Christ, God first disciplines, as Hebrews says, “Whom God loveth, he chasteneth.” He does not chasten the unbeliever, but He will chasten the believer. If chastening does not wake him up, God may take him out—the sin unto death, which Paul teaches. Believers have freedom and free exercise of will, but they should be aware that God will discipline obnoxious Christian behavior. God is the originator of discipline and knows how.