Listen, I tell you a mystery: We will not all sleep, but we will all be changed—
Hitherto he has included two things in his reasoning. In the first place, he shows that there will be a resurrection from the dead: secondly, he shows of what nature it will be. Now, however, he enters more thoroughly into a description of the manner of it.
Verse 51. Behold, I shew you. This commences the third subject of inquiry in the chapter--the question, what will become of those who are alive when the Lord Jesus shall return to raise the dead? This was an obvious inquiry, and the answer was, perhaps, supposed to be difficult.
Behold--Calling attention to the "mystery" heretofore hidden in God's purposes, but now revealed. you--emphatical in the Greek; I show (Greek, "tell," namely, by the word of the Lord, Th1 4:15) YOU, who think you have so much knowledge, "a mystery" (compare Rom 11:25) which your reason could never have discovered.