Then why does He say that those who are "alive and remain" shall be "caught up together with them"?
This essentially illustrates the who problem with this post - it ignores obvious flaws in its own doctrine, while focusing on a word study that doesn't even really say what it purports to say.
This passage says nothing about people being "left behind" -- I pray you read the whole of 1 Thessalonians
The famous rapture passage is found in 1 Thessalonians 4.15-17 and reads:
According to the Lords word, we tell you that we who are still alive, who are left until the coming of the Lord, will certainly not precede those who have fallen asleep. For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever.
This passage, when placed in the larger context of the chapter, is answering questions that Christians in Thessalonica had concerning death. What has happened to our loved ones who have died before the return of Christ to earth? What is theirs and our ultimate destiny? Pauls answer: bodily resurrection at the return of Christ to earth! Not an escape into the sky [see appendix below!].
In this passage, Paul borrows two specific images from the Old Testament that would have been familiar to Jewish converts and Gentiles who were familiarizing themselves with the Hebrew tradition. The first of these that Paul employs in the text has to do with Moses who comes down from Mount Sinai with the Law with the great blast of the trumpet.
The second image is taken from Daniel chapter 7 where the one like the son of man (or “human being” or “The Human One”) and the community he represents is vindicated over the enemies of the people of God. Clouds here symbolize the power and authoritative judgement of God about the rescue of his people. This idea now seems to be applied to Christians who are facing various forms of persecution.