1 Then I saw another sign in heaven, great and amazing, seven angels with seven plagues, which are the last, for with them the wrath of God is finished.
These 7 plagues which are the Bowls in Revelation 16 that will be brought by the angels of God, are the final judgments that will satisfy God’s righteous anger towards sinful humanity.
2 And I saw what appeared to be a sea of glass mingled with fire—and also those who had conquered the beast and its image and the number of its name, standing beside the sea of glass with harps of God in their hands.
“sea of glass mingled with fire” is a reference to God’s throne, see Revelation 4:6.
The believers who went through the great tribulation and did not accept the mark of the beast (666) or his name were given the honor to sing a song of praise in God’s presence.
3 And they sing the song of Moses, the servant of God, and the song of the Lamb, saying,
“Great and amazing are your deeds,
O Lord God the Almighty!
Just and true are your ways,
O King of the nations!
The song of Moses is a reference to having victory through God’s Mighty Power. If you go to Exodus 14, it takes you back to the time when God split the sea in two like two walls on either side. The Israelites passed through but the Egyptians were killed when the waters came back down on them. The LORD saved Israel that day and because of such a great deliverance from the Lord, Moses and the people sang a song of praise to God Almighty (Exodus 15).
In the same way, the believers who died in the great tribulation for not receiving the mark of the beast by being faithful to Jesus Christ through His power, they now show gratitude for their eternal rest in heaven and sing praises in God’s honor.
4 Who will not fear, O Lord,
and glorify your name?
For you alone are holy.
All nations will come
and worship you,
for your righteous acts have been revealed.”
Verse 4 could be in reference to Christ’s millennial reign
5 After this I looked, and the sanctuary of the tent of witness in heaven was opened
“the sanctuary of the tent of witness in heaven” refers to God’s Temple in heaven. See Revelation 11:19.