And I saw another SIGN in HEAVEN, GREAT and MARVELOUS,
This is the third sign John writes about: Sign 1 (Israel) is in Revelation 12:1 and Sign 2 (the red dragon) is in Revelation 12:3. This sign follows the harvesting of the earth. It shows the preparations for the pouring of the seven bowls of God's wrath upon the earth. Note that while the action takes place on earth, the sign is given in HEAVEN. This indicates that God is directing this sign.
The expression, GREAT and MARVELOUS, is used in verse 3 and is associated with God's works.
seven ANGELS who had seven PLAGUES, which are the last,
These seven angels probably are specifically designated with these PLAGUE judgments. They are attendants in the temple of heaven (verse 6). They understand how their pouring out the contents of each of their bowls will afflict the earth. In particular, one of them indicates a judgment against the great harlot (Revelation 17:1). At another time, one of them shows John the BRIDE of the Lamb (after the bowl judgments are completed).
We should be aware that with the sin curse brought upon the earth (Genesis 3:17-24), the cleansing/purging of evil HAD to happen because God is HOLY and RIGHTEOUS. This evil would not go unpunished.
because in them the WRATH OF GOD is FINISHED.
When I think about how powerful God must be to have created earth (and ultimately me), what kind of power (verse 8) is embedded in the WRATH OF GOD!
Believers and those who do not take the mark of the beast are NOT on the earth but rather are in heaven on the sea of glass (next verse) getting their harps ready to play the 'song of Moses'. The unbelievers will experience the PLAGUES contained in the seven bowls (Chapter 16). Many will die. Babylon gets its judgment at this time too (chapters 17-19).