That does not mean that the first resurrection (the resurrection of the righteous) cannot be in three phases.
please explain 1 Cor 15:23 then. But each in turn: Christ, the firstfruits; then, when he comes, those who belong to him.
I see 2 resurrections here: Jesus as the first one, and specifically called "firstfruits", and then ONE MORE, placed "when He comes".
Are there verses that Jesus comes in phases? Please show me if you believe that.
Finally, "those who belong to Him" obviously refers to EVERY saved person in history, because He is the Savior especially of those who believe. That would include everyone from Adam forward.
Why does the Bible compare the first resurrection to a Hebrew harvest?
It doesn't. You only imagine that was what Paul had in mind. Everyone in Paul's day understood what first fruits referred to. And Acts 26:23 specifically says that Jesus was "the first to rise from the dead", an obvious reference to receiving His imperishable glorified body.
Recall how many people were brought back to life BEFORE Jesus was resurrected: widow's son in 1 Kings 17:22, Lazarus, another widow's son, and lots of saints who came out of their graves at the crucifixion. Yet, Jesus is the FIRST human to receive a glorified body. None of the others did.
Because a Hebrew harvest was in three phases: (1) the first fruits, (2) the main harvest, and (3) the gleanings. And that is applied exactly to the first resurrection: (1) Christ the First Fruits, then (2) the Resurrection/Rapture which includes the entire Church, and finally (3) the resurrection of the Tribulation saints (Rev 20).
Nice try. But nope. Your 3 phases are not supported by Scripture. If you were right, the Bible would not refer to believers getting their glorified bodies in the singular.
The reason why Revelation 20 connects the first resurrection to the martyred saints is because that chapter also includes the second resurrection (the resurrection of damnation) before the Great White Throne Judgment.
There is no connection between "those who belong to Him" and the SECOND resurrection at all.