1 Corinthians 15:51

To confirm what he had said of this change, I. He here tells them what had been concealed from or unknown to them till then - that all the saints would not die, but all would be changed. Those that are alive at our Lord's coming will be caught up into the clouds, without dying, Th1 4:11. But it is plain from this passage that it will not be without changing from corruption to incorruption. The frame of their living bodies shall be thus altered, as well as those that are dead; and this in a moment, in the twinkling of an eye, Co1 15:52. What cannot almighty power effect? That power that calls the dead into life can surely thus soon and suddenly change the living; for changed they must be as well as the dead, because flesh and blood cannot inherit the kingdom of God. This is the mystery which the apostle shows the Corinthians: Behold, I show you a mystery; or bring into open light a truth dark and unknown before. Note, There are many mysteries shown to us in the gospel; many truths that before were utterly unknown are there made known; many truths that were but dark and obscure before are there brought into open day, and plainly revealed; and many things are in part revealed that will never be fully known, nor perhaps clearly understood.

— Matthew Henry's Concise Commentary (public domain)

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. 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 Th1 4:15, Th1 4:17, and with the apostle's argument here, which is that a change is necessary (Co1 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 (Heb 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."

— Jamieson-Fausset-Brown Commentary (public domain)