Revelation 1:19 is the key to understanding the time framework for The Book of Revelation: “Write the things which thou hast seen, and the things which are, and the things which shall be hereafter.”
This shows three divisions:
1) the things which thou hast seen (past)
2) the things which are (present)
3) the things which shall be hereafter (future)
Number 1 is Chapter 1: “the things which thou hast seen,” which includes the following things John had seen:
Verse 12: “And I turned to see the voice that spake with me. And being turned, I saw seven golden candlesticks.”
Verse 17: “And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me, Fear not; I am the first and the last.”
These verses preceded verse 19 and are “the things which thou hast seen” (i.e. the past).
Number 2 is Chapters 2 and 3, the Church Age, which is where we are time-wise. These chapters are “the things which are” (i.e. the present).
Number 3 starts with Chapter 4. The very first verse tells us that what happens then is that time period Jesus spoke of in Rev. 1:19:
“After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter” (Rev. 4:1).
Rev. 1:19: “the things which shall be hereafter”
Rev 4:1: “things which must be hereafter”
It can’t be any plainer that this is where the future begins, after the Church Age has ended; and the only way the Church Age ends is if the Church is no longer on the Earth – hence, the Rapture.
If the Church were still on Earth, then it would still be “the things which are.” The only way we can get to “the things which shall be hereafter” is if we have moved to AFTER “the things which are.” But that can’t be “after” if the Church is still here (i.e. “the things which are”).
In comparing Rev. 1:19 and Rev. 4:1, we see that Jesus made a stronger emphasis in the latter. In Rev 1:19, He said “shall be.” In that context, “shall” is the Greek μέλλω mellō which means:
μέλλω méllō, mel’-lo: a strengthened form of G3199 (through the idea of expectation); to intend, i.e. be about to be, do, or suffer something (of persons or things, especially events; in the sense of purpose, duty, necessity, probability, possibility, or hesitation).
Biblical usage includes: to be about; to be on the point of doing or suffering something; to intend; have in mind; think to.
But in Rev. 4:1, instead of “shall,” the Lord uses a stronger word, “must.” Remember that nothing in the Bible is there by accident. In the context of Rev. 4:1, “must” is the Greek δεῖ dei, which means:
†δεῖ deî, die; 3rd person singular active present of G1210; also deon deh-on’; neuter active participle of the same; both used impersonally; it is (was, etc.) necessary (as binding):—behoved, be meet, must (needs), (be) need(-ful), ought, should.
Biblical usage includes: it is necessary; there is need of, it; behooves; is right and proper; necessity lying in the nature of the case; necessity brought on by circumstances or by the conduct of others toward us; necessity in reference to what is required to attain some end.
The use of “must” in Rev. 4:1 is very strong, and carries the needfulness of “necessity.” So, that which happens in Rev. 4:1 “must” happen after the things of the present time. In other words, what happens in the future can only happen if the present time is done and over.
Therefore, the Rapture MUST happen in Rev. 4:1 to end the Church Age; otherwise, what happens in Rev. 4:1 is still the present time, which contradicts Rev. 1:19 and all rules of grammar and plain word sense