But we do not want you to be uninformed, brothers, about those who are asleep that you may not grieve as others do who have no hope. For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep. For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not recede those who have fallen asleep. For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God And the dead in Christ will rise first. Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord.
Paul wrote this passage to comfort those believers who had lost loved ones to death. They worried that their loved ones missed the resurrection by dying before the return of Christ. They saw their unbelieving neighbors grieve intensely, for they did not believe there was an afterlife for them. Paul said "don't do that". He pointed them to Jesus' resurrection. He was the first of many who would be raised.
In fact, when Jesus returns, he will bring with him those who have died in faith. That shows us that their souls are already with Jesus in heaven. Otherwise, he could not bring them with him when he returns from heaven. In addition, they will be the first to receive imperishable bodies. Then, those who are still alive at that time, will be raised and changed also. From that point, we will never be separated from the presence of the Lord.
It is appropriate to grieve the loss of one we love. After all, we miss them being with us. Death is not really natural It came as a result of sin. It is the penalty of sin. So we grieve. But we do not despair. We do not rail against God. We know that person is in a far better state than us: they are in heaven with Jesus. And we know that we will all be together with Jesus at his coming.
Therefore, we grieve, but with hope.