Translation
For if we believe that JESUS died and rose again, so also [we believe] that THEOS will bring with Him those that
have fallen asleep through JESUS.
Paraphrase
For if we believe that THE SAVIOR died and rose again, we need to also believe that THE CREATOR AND OWNER OF ALL THINGS will bring with Him those who have died while in good standing with THE SAVIOR.
THIS IS A SUMMARY STATEMENT
Paul’s focus here is on those who have already died in Christ, the ones that the Thessalonian believers are mourning for. He wants them to know that those who have died in Christ will rise first when Christ returns (v 16). The statement here in vs 14, indicating that Jesus will bring with Him those who have fallen asleep in Him, means that they will rise first to meet Christ in the air as He comes to earth to begin the process of meting out punishment and rewards, and they will accompany Him from their meeting place in the air back to earth with Jesus. The believers that are alive at that time will do the same, the difference being one of timing; they will join Jesus in the air after those who have already died (fallen asleep) in Him and also accompany Him back to earth (see my comment after verse 18 for more on this topic of meeting Him on His way to earth).