Why do believers need new bodies?
Believers need new bodies because the old body cannot enter glory. This corruptible body must put on incorruption, and this mortal body, prone to die, must put on immortality. A new body is required to make believers fit for eternity in God's presence.
Les explains that Paul calls this change a secret or mystery, because Scripture had given no indication of a group of living people suddenly translated until Paul. Jesus never mentioned it, and the Old Testament never mentioned it, but Paul does. Paul writes that believers shall not all sleep or die physically, but all shall be changed—in a moment, in the twinkling or blink of an eye, at the last trump. The dead shall be raised incorruptible, and living believers shall be changed suddenly. The reason, Les says, is that we cannot go to glory in this old body. This corruptible must put on incorruption, and this mortal body, prone to die, must put on immortality. We have to be made fit for eternity in God's presence and be given this new body.