Charles Ellicott Commentary 1 Corinthians 15:51

"Behold, I tell you a mystery: We all shall not sleep, but we shall all be changed," — 1 Corinthians 15:51 (ASV)

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 until now has been kept concealed, but is now revealed (Romans 11:25; Ephesians 3:3–5).

We shall not all sleep, but we shall all be Changed.—There is a considerable variety of readings here 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.

This change from the earthly to the spiritual body is absolutely necessary. For some, it will come through the ordinary process of death; for 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.