Why does the corruptible body have to put on incorruption?
The corruptible body must put on incorruption because the believer is going to live for eternity and therefore needs a body fit for eternity. The mortal, prone to die, must also put on immortality so that it can never die, and death is finally swallowed up in victory.
Les explains that in these verses Paul is dealing with the resurrection of the Body of Christ, the Church Age believer, not Old Testament believers or the tribulation. The dead will be raised first with their new body, and then those alive will be changed. Why? Because this corruptible body is prone to death, but we are going to live for eternity, so we must have a body fit for eternity. Likewise, this mortal body is prone to die, so it must put on immortality so we can never die. Then, when the corruptible puts on incorruption, death is finally swallowed up in victory, and we will never again face the prospect of death.
In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
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