We are told that the rapture will occur in the twinkling of an eye. We are also told that the dead in Christ will rise first. Now, the twinkling of an eye is not very long. What would be the purpose of those who died in Christ to be raised a fraction of a second faster than others? God gave man a last chance to change their hearts before the flood. The door of the ark was left open for a few days before it was shut by God. Just think of a short time period between the two raptured groups. It would give the living christians an incredibly powerful and bold chance to maybe witness to close relatives.
Paul was responding to a fear the Thessalonians had that those who died before the rapture will miss out on all the promises that accompany it (1 Thes. 4:13). In effect Paul was saying, “Not only will they not miss out, they’ll be the first in line to receive them.” That was his purpose in saying the dead will rise first.
Also, when the rapture takes place the full number of the Church will have come in (Romans 11:25). No one else will be part of it. The final witness we’ll provide to friends and family is our disappearance.