Why shouldn't a believer doubt their salvation?
A believer should not doubt salvation because the Holy Spirit baptizes every believer into Christ's body, guaranteeing eternal destiny. At faith in the Gospel, one is forgiven, redeemed, justified, translated, reconciled, and glorified. Since God has declared it, the believer rests on His Word by faith rather than succumbing to the devil's doubts.
Les explains that the Bride of Christ is that consortium of all the believers of the Church Age, who have been placed into the body by the Holy Spirit. He notes that many church roles may include unsaved people, because we cannot look on the heart or determine who is saved and who is lost—only God can look into that area. But into the body of Christ there are no unbelievers; no unsaved person is ever baptized by the Holy Spirit into the body of Christ. That baptism guarantees your eternal destiny, and it happens when you believe the Gospel, with all attendant aspects of salvation done at the same time. So Les asks, how can you doubt if you have been forgiven, redeemed, justified, translated, reconciled, and glorified? These are taken by faith: God has said it, we believe it, and we rest on it. The old devil likes to pooh-pooh the whole idea, but we come right back and claim it.