What does 1 Corinthians 15:51 mean?

Some in Corinth claimed there was no resurrection from the dead for Christians. Paul has disputed that almost entirely, though he has agreed on at least one point. Namely, that there's no way corrupt, dying "flesh and blood" bodies can possibly inherit the kingdom of God. Put another way, our naturally-born bodies cannot exist in God's presence in heaven for eternity. What the confused Corinthians don't understand is the mystery Paul reveals in this verse. It's the mystery he has been revealing throughout chapter 15. In a word, this is "transformation." God will, in fact, transform the "natural" bodies of Christians into glorified, or "spiritual" bodies (1 Corinthians 15:44) that will inherit God's kingdom, after all.

Paul begins here by announcing, "Behold!" to indicate what he says has great meaning. The mystery is revealed in this and the following verse. Paul has already said that the resurrection of the dead will take place when Christ comes back for those who belong to Him (1 Corinthians 15:23), as Jesus said He would do (John 14:2–3).

However, some believers will not yet have died when Christ returns. How will they step into eternity if they are still in their sin-stained "natural" bodies? Paul's answer is that whether a person dies and their body is "sown," (1 Corinthians 15:37), or they are taken in the rapture (1 Thessalonians 4:15–17), all born-again believers will experience change when they step into eternity. The living will be transformed, along with those who are resurrected from the dead. All believers will be physically remade into their glorified bodies at the return of Christ (1 John 3:2).