Revelation uses the verb “descend” (katabaino) ten times, often with the specification that something descends “from heaven.” Cities come down from the sky, and angels, and dragons, and fire. The ten uses of the verb are in a fairly neat chiastic pattern:
A. New Jerusalem will descend, 3:12
B. An Angel descends to feed John a book, 10:1
C. Satan falls, 12:12
D. The land beast makes fire fall from heaven, 13:13
C’. Hailstones fall, 16:21
B’. Angels descends, 18:1; 20:1
D’. Fire falls to destroy Satan, 20:9
A’. New Jerusalem descends, 21:2, 10
The fire of 20:9 stands interrupts the pattern, and the connection between C and C’ is tenuous, perhaps non-existent. It is possible to see an indirect connection. Satan’s fall to earth allows him to raise up beasts and to establish his throne on earth. Hailstones fall because the firmament is being shattered, as the Lord’s throne descends from heaven, ultimately overthrowing the dragon’s throne.
Even if the chiasm doesn’t quite work, Revelation is neatly bookended by three references to new Jerusalem coming from heaven (3:12; 21:2, 10), in a 1 + 2 pattern. And the central action sequence of the book is also neatly bookended with three references to angels descending (10:1; 18:1; 20:1), in something like a 1 + 2 pattern.
These two similar clusters work together. The initial promise from heaven is of a new city, and that promise is made good in chapter 21. Within that frame is the story of how the way is prepared for the city to descend: An angel descends to feed John the book that he will speak in prophecy, and then angels later descend to announce the fulfillment of the prophecy, in the overthrow of Babylon (18:1) and of Satan (20:1). So we have this structure:
A. Promise of new Jerusalem.
B. John commissioned to prophecy (angel descends).
B’. Babylon, the old city, falls (angel descends).
B”. Satan, the seducer, is chained (angel descends).
A’. The promise of new Jerusalem fulfilled.
Perhaps we are to understand that John’s prophecy is not only about the fall of Babylon and Satan, but is God’s weapon in achieving victory of the city and the Satan. By prophesying, John becomes one of the means by which the city falls.