{A mystery} (must(8872)ion). He does not claim that he has
explained everything. He has drawn a broad parallel which opens
the door of hope and confidence. {We shall not all sleep}
(pantes ou koim(8874)h(8873)ometha). Future passive indicative of
koimaomai, to sleep. Not all of us shall die, Paul means. Some
people will be alive when he comes. Paul does not affirm that he
or any then living will be alive when Jesus comes again. He
simply groups all under the phrase "we all." {But we shall all be
changed} (pantes de allag(8873)ometha). Second future passive
indicative of allass(935c). Both living and dead shall be changed
and so receive the resurrection body. See this same idea at more
length in  1Th 4:13-18 .