51 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
All Christians who have died after 70 A.D. immediately enter heaven. There is no sleeping anymore.
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.
The same seventh trumpet in Rev 11 that refers to the falling of Jerusalem and the old heavens and earth. We see the resurrection described in the last trumpet of Rev 11: 15-19. That is the last trumpet of 1 Cor 15: 52.
53 For THIS corruptible must put on incorruption [this is "the dead in Christ"], and THIS mortal must put on immortality [this is the "we which are ALIVE and remain unto"].
54 So when THIS corruptible ["the dead in Christ"] shall have put on incorruption, and THIS mortal [the "we which are ALIVE and remain unto"] shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
What does Romans 5 say? Death reigned through Adam. Life comes through Christ. The corruptible putting on incorruption is the transition from the old covenant/fallen Adamic world to the new covenant/Christ's kingdom world. It is the passing away of the old heavens and earth and the beginning of the new heavens and earth, Christ's kingdom. Passing from corruption to incorruption is covenantal and corporate language.
Christ brought His kingdom with Him in 70 A.D. as Matt 22: 1-14 and Luke 21: 31 and many other passages say. Rev 21, which describes the new heavens and earth does not describe a re-made universe. It describes the church and Christ as its light.
Death was swallowed up in that moment. Spiritual death, not physical death, when the OT and 1st century saints were raised. This is not a transformation of physical bodies.