Those Who Won't Sleep
Death is as natural as life. In fact, in due time, death is necessary for life - eternal life.
"9:27 And as it is appointed unto men once to die, but after this the judgment:
While awaiting their future resurrection, those who die are in a state of inert oblivion; total peace in which there is no awareness or memory (the "memory of them is forgotten" in the verse below refers to the dead person's memory, since people who die are in the memory of those still living).
"9:5 For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten." (Ecclesiastes 9:5 KJV)
"We shall not all sleep, but we shall all be changed"
On the day of Christ's return, all of God's people who have lived and died through all time will be resurrected from the dead (the rest of humanity, who did not receive their calling to understanding will be resurrected at the end of the 1,000 years - see When Will You Be Judged?).
"4:14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
Notice carefully in verse 17, quoted above, that along with the dead who will be resurrected from their graves, "we which are alive and remain shall be caught up together with them in the clouds." Apart from those who have died and will be resurrected, there remains the matter of those of God's people who will be alive on the day of Christ's return. They will be unique in that, rather than dying as all others do (whether of illness or injury), they will be instantly changed to spirit as their physical bodies die. They will be dead, then alive, before their body hits the ground - a consciously-instantaneous change that will however seem to be no different to those who have been dead for centuries (see the Fact Finder question below to understand how the dead, from their own sense of awareness, will seem to have gone instantly from death to resurrection).
"15:50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
Fact Finder: How is it that Christ could seem to return now?