Who are those Paul's describes as “asleep” in 1Thessalonians 4?
Paul writes to the church in Thessalonica saying:
In v.13 Paul uses the word “asleep” as a polite way of describing death, in the same way we might say someone has “passed away” rather than saying the person has died. Therefore, those who are “asleep” in 1Thess 4 are those Christians who have died. By the context, we see Paul was referring to believers who die prior to Christ’s coming for the Church at the Rapture.
For a complete explanation of this passage, please read "How do we know there will be a Rapture."