If the dead are already in heaven, how do they rise at the second coming?

1 Thessalonians 4:16 says: > For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: It's my understanding that most people in the Western world believe in Christian immortalism though. If the dead are already in heaven, how are they rising here?