What does it mean that we shall not all sleep but we shall all be changed?
It means not every believer will experience physical death before Christ's return; the generation alive at His coming for the Church will be transformed. Their bodies will be changed into resurrection bodies like Christ's: spiritual, flesh and bone without blood, activated by the Spirit, not requiring death.
Les explains that this was a secret God first revealed to Paul, something never before alluded to, hinted at, or spoken. Toward the end of time, there will be a group of believers who will not die physically. Paul writes, 'We shall not all sleep or die physically, but we as believers shall all be changed.' That change means their resurrected body will be fashioned after Christ's resurrection body: it will be like our physical body but different—a spiritual body without blood, flesh and bone but no blood, activated by the Spirit. Les notes Paul writes as though it would happen in his lifetime, so this teaching calls it the imminent coming of Christ for the Church. It may be tonight, tomorrow, or ten years from now, and believers are to expectantly look for it at any time, because no set prophetic program must be fulfilled first.
Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,