A mystery (musthrion). 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 (pante ou koimhqhsomeqa). 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 (pante de allaghsomeqa). Second future passive indicative of allassw. Both living and dead shall be changed and so receive the resurrection body. See this same idea at more length in 1 Thessalonians 4:13-18 .