The letter has a whole chapter mainly about the Second Coming of Christ. Someone had told the Thessalonians that Christ had already returned. Paul says this is not true. (chapter 2, verses 1 to 12), He tells the Thessalonians that a great tribulation (time of trouble) must occur before Christ's return. Since this has not yet happened, Christ cannot have returned yet. He then gives thanks that his readers were chosen by God for salvation and saved by His grace through faith, and so would not be deceived by false teachings. (Chapter 2, verses 13 and14)