This is why the elders are rejoicing. Anything in chapter 12 has nothing to to with it. Let's examine the sequence in context:
Here we see the 3rd woe announced as coming quickly
Rv 11:14-15 The second woe is past; and, behold, the third woe cometh quickly.
John wrote down and recorded everything he was shown and told. Which is presented to us as the book of Revelation.
Revelation 11:14 The second woe is past;
and, behold, the third woe cometh quickly.
Revelation 11:14 is
a notation that John makes to the readers of Revelation, like us. John is noting to the readers of Revelation, that the second woe, what it will be, has already been shown. And that the readers will soon be shown what the third woe will be. Which happens in Revelation 12:12.
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So back to the text of Revelation 11. All the events associated with two witnesses have just been presented for everyone in heaven to see, in Revelation 11:3-13.
And next, to everyone in heaven, they see and hear the seventh angel sound his trumpet in Revelation 11:15. And great voices in heaven proclaimed - "The kingdoms of this world are become
the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever."
So what does that mean ? It means that the time for the kingdom of God to brought to earth has come. In the days (not day) of the seventh angel sounding his trumpet.
Then, the twenty four elders, excited, react to the news, in Revelation 11:16-18.
And then there in heaven, as the twenty four elders look on - in Revelation 11:19 And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail.
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So the main thing I am trying to get across in this post is that Revelation 11:14 is a notation that John makes to the readers of Revelation. We've read about what the second woe will be back in Revelation 9. And that we will soon read about what the third woe will be in Revelation 12.