In His return, Christ will bring with Him those believers who have gone ahead of us in glory. We who are alive at the time of His return, however, will be changed as the apostle Paul also said in 1 Corinthians 15:51-52,
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.
That’s the reason why Paul could conclude the 1 Thessalonians passage with an exhortation, "Therefore encourage one another with these words" (v.18). We should not think that our believing loved ones who died will be forgotten. Christ will bring them with Him when He returns again in glory.
In His coming (or presence, from Greek "parousia"), Christ will 'come down from heaven' with spectacular phenomena: "...with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God" (v.16). Definitely everyone alive at that time will see and hear these sounds.
But how will the second coming of Christ be visible and audible to all? Dr. Greg Beale has a beautiful description of this descent of Christ in His 'parousia.' In his commentary on 1-2 Thessalonians (p.138) he said,
Comparing other descriptions of Christ's coming, it is apparent that 'motion' from heaven down to earth may not be the precise way in which Christ manifests his end-time presence [parousia]. Revelation 6:14 refers to the end of the present cosmos in terms of 'a scroll that has been split and each of the two halves then rolled up'. If John were living today, he might use an analogy of a stage curtain with pictures on it, which is drawn from both sides to reveal the actors behind it. In short, the present physical reality will in some way disappear and the formerly hidden heavenly dimension, where Christ and God dwell, will be revealed (see further Rev. 11:19; 19:11; 21:1-3).
In other words, when our Lord Jesus Christ returns, "he will not descend from the sky over Boston or London or New York City or Hongkong [or Davao City] or any other localized area. When he appears, the present dimension will be ripped away, and Christ will be manifest to all eyes throughout the earth (see Mt 24:27). Just as one can lay flat a map of the whole world and see it all at one glance, so Christ will appear and be able to behold humanity at one glance and they him" (Beale, p.138-139).
How will this happen "in literal geographical terms is certainly unclear, but the answer lies in recalling that a new dimension will break into the old physical dimension, and the possibilities of new kinds of perception and of existence beyond present understanding will then be realized" (Beale, p.139).