Paul speaks about the resurrection, "According to the Lord's own word" in 4:15. Either Jesus spoke of this while he was alive and it was not written in the Gospels, being left to the oral tradition, or Paul had a personal revelation of it. Specifically he says, "We tell you that we who are still alive, who are left till 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 (1 Thess 4:15-17).
So, when Christ returns, the dead in Christ will rise first and then those who are still alive will rise to meet Christ in the air. If those who are alive at that time get to see the rising bodies of those who have died, it will be quite a sight. But I don't think Paul was trying to paint a picture so much as trying to reassure the families of the deceased that there would be a bodily resurrection of both the dead and the living, and they would be reunited with their loved ones. We don't know exactly when that will happen, but happen it will (5:1-3).
Let us pray. Jesus,you were crucified, you died and you were buried. You rose on the third day. You ascended into heaven and are seated at the right hand of the Father. You will come again in glory to judge the dead and the living and your kingdom will have no end. We look forward to the resurrection of the dead and to being with you in the world to come. Because of you we have hope.