What is the mystery that some believers will not die physically?
The mystery is that one group of believers will not die physically but will remain alive at the culmination of the church age. At the rapture, they will be suddenly changed in a split second and caught up with resurrected believers to meet the Lord in the air, as revealed to Paul.
Les teaches that this is a Pauline doctrine found in 1 Corinthians 15:51, where Paul calls it a mystery—a secret never before revealed. This mystery concerns a group of believers at the end of the church age who will not die physically but will be alive when everything comes to a culmination. At that moment Christ will leave glory, come to the atmosphere, the trumpet will sound, the archangel will shout, and the dead believers of the church age will be raised. Then living believers will follow in a split second, meeting the Lord in the air. Les explains that this is hard to comprehend and takes faith, which is why unbelievers scoff at it.
Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,