Let no one deceive you in any way. For it will not be, unless the departure comes first, and the man of sin is revealed, the son of destruction,
Paul wants to guard the Thessalonians lest they be deceived by the false teaching concerning the coming of Christ. He lists three ways they could be deceived and now directly warns them against the possibility. If anyone tells you that the end of the world, or the coming of Christ, is imminent, is a false teacher because there must "come a falling away first."