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,

 The general picture is this. Paul was telling the Thessalonians that they must give up their nervous, hysterical waiting for the Second Coming. He denied that he had ever said that the Day of the Lord had come. That was a misinterpretation of his words which must not be attributed to him; and he told them that before the Day of the Lord could come much had still to happen. 