Answer: When the Lord Himself comes down from heaven, the dead in Christ will rise first. Those people in Christ who are still alive and are left here on earth will be caught up together with them in the clouds. All of the Lord's people will meet Him in the air, and we will be with the Lord forever, (verses 16 and 17).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
Acts 1:9-11: "After He [Jesus] said this, He was taken up before their very eyes, and a cloud hid Him from their sight. They were looking intently up into the sky as He was going, when suddenly two men dressed in white stood beside them. ‘Men of Galilee,' they said, ‘why do you stand here looking into the sky? This same Jesus, Who has been taken from you into heaven, will come back in the same way you have seen Him go into heaven.'"
Revelation 1:7: "Look, He is coming with the clouds, and every eye will see Him, even those who pierced Him; and all the peoples of the earth will mourn because of Him. So shall it be! Amen."
