Revelation 15The bowl judgments are coming and with that, God wrath will be completed. The apostle John, in heaven, sees 7 angles with the last seven plagues. He then see that sea glass that is now mixed with fire. Again the format takes on angels and people in heaven praising God and then the judgments are poured out. The Sea of glass could be part of the tabernacle which the priests washed themselves with. Now its mixed with fire to purify, not just clean. Jesus himself references a baptism of fire
15 Then I saw another great and awe-inspiring sign in heaven: seven angels with the seven last plagues, for with them, God's wrath will be completed. 2 I also saw something like a sea of glass mixed with fire, and those who had won the victory from the beast, his image, and the number of his name, were standing on the sea of glass with harps from God. 3 They sang the song of God's servant Moses, and the song of the Lamb:
Great and awe-inspiring are Your works, Lord God, the Almighty;
righteous and true are Your ways, King of the Nations.
4 Lord, who will not fear and glorify Your name?
Because You alone are holy,
because all the nations will come and worship before You,
because Your righteous acts have been revealed.
5 After this I looked, and the heavenly sanctuary—the tabernacle of testimony—was opened. 6 Out of the sanctuary came the seven angels with the seven plagues, dressed in clean, bright linen, with gold sashes wrapped around their chests. 7 One of the four living creatures gave the seven angels seven gold bowls filled with the wrath of God who lives forever and ever. 8 Then the sanctuary was filled with smoke from God's glory and from His power, and no one could enter the sanctuary until the seven plagues of the seven angels were completed.
Matthew 20:23-24Given the fact that right after the reference of the sea of glass mixed with fire, we see those who had won victory over the beast, so indeed they had purified themselves in the tribulation. Here these martyred tribulation saints are singing the song of Moses like what happened when God delivered his people through the red sea but when the Egyptians tried to follow the red sea came crashing down on them, killing all of who wanted to harm God’s chosen people.
23 He told them, "You will indeed drink My cup. But to sit at My right and left is not Mine to give; instead, it belongs to those for whom it has been prepared by My Father."
Given the fact that verse 5-8 are fairly cut and dry, all that can be said is a recap of what the Bible already states but I do like how the Bible knowledge commentary describes the events in a way I could not.
Revelation 15:5-8
15:5-8. As John continued to look at the heavenly vision, he saw the temple described as the tabernacle of Testimony. The allusion to a temple in heaven seems to be the heavenly counterpart of the earthly temple. As it was opened, the seven angels with their plagues exited from it. The clean, shining linen of the angels indicates their purity, and the golden sashes around their chests point to the glory of God.
John saw one of the four living creatures give the seven golden bowls filled with the wrath of God to the seven angels. When this was done, smoke filled the temple, making it impossible for anyone to enter it until the seven plagues were poured out on the earth (cf. Exodus 40:34-35). Taken as a whole, Revelation 15:5-8 presents a fearful picture of impending divine judgment on a wicked world. The judgments which are to be poured out (chap. 16) fully justify this ominous introduction.
To summarize, we have a dramatic pause, which provides a picture of heaven, where the martyred tribulation saints sing the song of Moses and the song of the Lamb. Praising God for who he is and it appears thankful that his wrath will be completed. God’s reign in the millennial kingdom is around the corner but first for the unbelieving, Christ rejecting world, they must endure the seven bowl judgments and while their fate is sealed if they’ve taken the mark of the beast there still may be some who have not and perhaps through this wrath, they may realize that God wants something better for them, if only they accept his gift of salvation.
God wants fellowship with his creation but he will not force himself on people, so its reasonable to assume that those people who reject God now and choose not to fellowship with him now do not wish to spend eternity with him. The alternative is scary because they must then pay for their own failings, sins and acts of wickedness on their own instead of leaning on the act of Christ who took on their punishment, if only they’d accept it. Regardless, people have a choice, God gave us free will and so they are free to go their own way.