Jamieson, Fausset and Brown on 1 Corinthians 15:51

51. 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, 1Th 4:15) YOU, who think you have so much knowledge, "a mystery" (compare Ro 11:25) which your reason could never have discovered. Many of the old manuscripts and Fathers read, "We shall all sleep, but we shall not all be changed"; but this is plainly a corrupt reading, inconsistent with 1Th 4:15, 17, and with the apostle's argument here, which is that a change is necessary (1Co 15:53). English Version is supported by some of the oldest manuscripts and Fathers. The Greek is literally "We all shall not sleep, but," &c. The putting off of the corruptible body for an incorruptible by an instantaneous change will, in the case of "the quick," stand as equivalent to death, appointed to all men (He 9:27); of this Enoch and Elijah are types and forerunners. The "we" implies that Christians in that age and every successive age since and hereafter were designed to stand waiting, as if Christ might come again in their time, and as if they might be found among "the quick."