Does Paul call the 1 Thessalonians 4:13 teaching a mystery?
No, Paul does not explicitly call the 1 Thessalonians 4:13 teaching a mystery. He writes, "I would not have you to be ignorant, brethren," concerning those who have died. The text could have said "concerning a mystery," but Paul does not; it is simply read as written.
Les explains that 1 Thessalonians 4 is another extension of the revelation of the mystery, which he refers to as being caught up. He notes that Paul's language is much the same as in 1 Corinthians 15. In verse 13, Paul says, "I would not have you to be ignorant, brethren," and Les observes Paul could have just as well said "concerning a mystery," but he doesn't, so it is read as it is in the text. Les also mentions that though the word "rapture" is not in the English Bible, there are two words that mean the same thing.
But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
More Questions
Is 1 Thessalonians 4 part of the mystery of the rapture even though it doesn't use the word mystery?
First Thessalonians 4 does not use the word 'mystery,' but Paul is talking about the same thing, so it too is part of the mystery—what is called the outcalling or the rapture of the church.
Why does Paul write 'I would not have you to be ignorant' in 1 Thessalonians 4:13?
Paul says 'I would not have you to be ignorant' in 1 Thessalonians 4 because the truth about those who have died had never been revealed before; Paul was the first to reveal it.