Does a believer who is sinning deeply lose their salvation if God takes their life?
No. Scripture says that when God takes such a believer out of physical existence as discipline for deep, open sin, that believer does not lose salvation. The sin unto death ends earthly life, not eternal life; the church is to discipline for restoration, not condemnation.
Les teaches that this concerns a believer who goes into deep, open sin that people generally know about. God may take these kind of believers out of their physical existence, but they are not going to lose their salvation. He connects this with 1 John 5:16, where John speaks of a sin not unto death and also a sin unto death. In the case not unto death, a believer can ask and God gives life. In the deeper case, God has mandated first and foremost a disciplinary action on the part of the church to restore such a one to fellowship. The believing community is never even to pray for the death of someone like that.