What does it mean that Christ's coming for the Church is imminent?
It means this event could occur at any moment—tonight, tomorrow, or years from now—and believers are to expectantly watch for it. No set prophetic program must be fulfilled before it takes place, since it was revealed as a secret not previously spoken.
Les explains that this imminent coming means it may be tonight, it may be tomorrow, or it may be ten years from now, but believers are to expectantly look for it at any time. He notes there is no set prophetic program to be fulfilled for this event to take place. Paul writes as though it would happen in his lifetime. The secret revealed is that a group of believers will not die physically but will all be changed, with a resurrected body fashioned after Christ's resurrection body—flesh and bone without blood, activated by the Spirit.
Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,