Two groups become one when they are changed and presented together in the air at the Return of Christ.
[Group 1] those that die before Christ's Coming.
[Group 2] those that would remain alive unto His Coming.
[Groups 1 + 2] together
CHRIST ~to those who actually heard Him, that first generation of believers:
John 11: 25-26 Jesus said to her, "I am the resurrection and the life. He who believes in Me, though he may die [Group 1], he shall live. 26 "And whoever lives and believes in Me [Group 2] shall never die. Do you believe this?''
Group 1: whoever believes and dies; shall live [in a new form, a spiritual body].
Group 2: lives and believes; Never die; shall never die [but still be "changed" into the same new form that Group 1 obtains, a spiritual body].
PAUL ~to those whom his letters were addressed, that first generation of believers:
1 Cor. 15: 35. But someone [of Group 2] will say, "How are the dead [Group1] raised up? And with what body do they [Group 1] come?''
1 Cor. 15:51. Behold, I tell you a mystery: We [Groups 1 + 2] shall not all sleep [like Group 1], but we shall all be changed [Groups 1 + 2] 52. in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead [Group 1] will be raised incorruptible, and we shall be changed [Groups 1 + 2].
2 Cor. 4:12-14 So then death is working in us [destined for Group 1], but life in you [destined for Group 2]. 13. But since we have the same spirit of faith, according to what is written, "I believed and therefore I spoke,'' we also believe and therefore speak, 14. knowing that He who raised up the Lord Jesus will also raise us [Group 1] up with Jesus, and will present us with you [Groups 1 + 2].
1 Thes. 4:16. For the Lord Himself will descend from heaven with a shout,with the voice of an archangel, and with the trumpet of God. And the dead in Christ [Group 1] will rise first. 17. Then we who are alive and remain [Group 2] shall be caught up together with them [Group 1] in the clouds to meet the Lord in the air. And thus we shall always be with the Lord [Groups 1 + 2].
Note also that the events that happen to both groups together [Groups 1 + 2] happen AFTER the resurrection of the dead [Group 1].
This is affirmation that the "Resurrection of the Dead" cannot be seen as simply a corporate resurrection or an individual realization of salvation. It is also an indication that "changed" (glorification) is something that follows resurrection (just as, also, damnation).