"We shall not prevent them which are asleep" means we won't stop the dead from rising

"We which are alive and remain unto the coming of the Lord shall not prevent them which are asleep" (1 Thessalonians 4:15, KJV). The Latin root, praevenire, means literally "to come before," and that older sense, "to precede" or "to anticipate," is exactly the meaning at work here: those still living at Christ's return won't get there ahead of those who have already died. The modern sense of "prevent" (to stop something from happening) had not yet become the word's primary meaning in 1611.

Read with the modern sense, the verse can seem to promise that the living won't obstruct the dead in some way, which isn't the point being made at all. Every modern translation renders this with "precede" or "go before" specifically to close that gap.