It is often stated that the Rapture must come before the Great Tribulation because the verses that describe this event end with instructions to encourage one another.
Brothers and sisters, we do not want you to be uninformed about those who sleep in death, so that you do not grieve like the rest of mankind, who have no hope. For we believe that Jesus died and rose again, and so we believe that God will bring with Jesus those who have fallen asleep in him. According to the Lord’s 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. Therefore encourage one another with these words. I Thess. 4:13-18
From these verses, the argument is made that since Paul ends his teaching by telling the churches to encourage one another, the Rapture must come before the saints are killed in the Great Tribulation, otherwise, there would not be anything to be encouraged about. The mistake in this argument is in the assumption that when Paul speaks of encouraging one another he is referring to the Great Tribulation, when, in fact, he is referring to the Resurrection of the Dead. This is made evident in the opening verse of this passage, which identifies the subject of Paul’s instruction as the Resurrection of the Dead.
Brothers and sisters, we do not want you to be uninformed about those who sleep in death, so that you do not grieve like the rest of mankind, who have no hope. 1 Thess. 4:13.
From this opening statement, we learn that Paul was writing to those who had no hope that the dead would be resurrected. These uniformed brothers and sisters believed that when the Lord returned, the living would be caught up to meet Him but those who had died would not. Paul comforted these Believers by informing them that when Christ returned, the dead in Christ would rise first. Accordingly, when Paul told the church to encourage one another, he was referring to encouraging those who were mourning the dead and who did not understand that the dead would be raised before the living were caught up. As an encouragement concerning the Resurrection of the Dead, these verses are not implying that Believers will be Raptured before the Great Tribulation, which is not even mentioned in these verses.
To read about the timing of the Rapture, click here: The Timing of the Rapture