Behold, I shew you a mystery. — It is better to take these words as referring to what follows rather than (as some have done) to the preceding statement. A mystery means something which up to this time has been kept concealed, but is now made manifest (Romans 11:25; Ephesians 3:3-5).
We shall not all sleep, but we shall all be Changed. — There are here a considerable variety of readings in the Greek, but the text from which our English version is taken is probably correct. The Apostle believed that the end of the world might come in the lifetime of some then living. We shall not all, he says, necessarily sleep, but we shall all be changed. The change from the earthly to the spiritual body is absolutely necessary. To some it will come through the ordinary process of death; to those who are alive at Christ’s advent it will come suddenly, and in a moment. The dead shall be raised, but we (the living) shall be changed.