One more note: Paul had spent less than two months with these new believers, yet the departure event was so significant a reality they were to watch for that he had to deal with their questions regarding the event in two letters to them. That's for those who think the notion of the snatching away is not important for believers. It is the foundation of the Biblical principle of imminency.
Try reading the actual text. By context, Paul is talking about the day when Christ comes to gather the church. And he says, "that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition"
Context, context context.