andBut 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 precede 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. Therefore encourage one another with these words. (1 Thess 4:13-18 ESV)
Notice both passages speak of the same event, the resurrection of the Christians who have died. And both speak of some Christians who are alive at the time of this resurrection. Those who have died will be raised imperishable, and those who are alive at the time will instantly be changed from a mortal state to an immortal state. I understand this to be what is called "The first resurrection"Behold! I tell you a mystery. We shall not all sleep, but we shall all be changed, in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised imperishable, and we shall be changed. For this perishable body must put on the imperishable, and this mortal body must put on immortality. (1 Cor 15:51-53 ESV)
As I understand it, those who are caught up to MEET the Lord in the air will return with him to the earth in his triumphant coming. I understand that the word translated as "meet" was used of persons leaving a king's city to welcome their triumphant king who was away to war, back into his city, and to return with him to that city.
As I see it, the millenium will follow, and those immortal persons will rule and reign with him here on earth. Mortals who live on the earth at that time will be long-lived, and many of them will become Christians. At the end of the millenium the second resurrection will take place, both of the non-Christian dead, and of those Christians who will have died during the millenium. It may even include those prior to Christ's return who have not lived their lives as overcomers.
I realize that some of what I have said is speculation. I have tried to put it all together in such a way as to make sense of it all. However, if I am totally wrong in my understanding of these passages, it won't bother me at all. In my opinion, beliefs about future events should not take priority in the Christian life.