Then I beheld another sign in the sky, extraordinary and marvelous: seven angels having seven calamities, uttermost and extreme because in them the passionate wrath of God was fulfilled, brought to completion.
And I saw what was like a glassy sea having been mingled with a fire, and the ones prevailing from out of the dangerous beast and from out of the image of it and from out of the number of its name were being made to stand upon the sea of glass, having the harps of God.
And they are singing the song of Moses, the slave of God, and the song of the Lamb saying, “Great and wonderful are Your works, Lord God the Omnipotent: Just, Righteous, and True are Your ways, the Ruler of the Ages, the Universe.
“Who would not be seized with fear, Lord, then give glory to Your name, that alone is holy? Because every people group will have come and will lie prostrate in homage in Your presence, because Your righteous acts and judgments are made manifest.”
Then, with these things I also beheld the temple of the habitation of the testimony opened in heaven.
Revelation 15:1-5

It seems John was gazing up into the sky, or perhaps he was in a transported state in heaven. Either way, what came before him now took his breath away. Seven angels filled his view, and each of them had, or held in some way, the final disasters that would mark the completion of God’s passion, the cleansing wrath God had held back until now, the appointed hour.
Glassy Sea
Yet, even as John stared round-eyed with horrified wonder at what was about to be unleashed, he realized there was a vast sea spread before him, a sea like glass, mixed with fire. Perhaps John, sitting high up in the cliffs of the Isle of Patmos, had seen the Aegean Sea in this manner, on a calm, clear summer’s dawn, the water as clear and smooth as a mirror, and the sun’s golden-red rays illuminating the ocean as though a fire opal were glistening before him.
Just such a dazzling sea now appeared, and even as John watched, a myriad of people also came into view. John knew within himself who they were—believers who had taken every warning to heart, who had suffered and endured, who had remained faithful to Jesus. They had prevailed, and now they were being strengthened,they were being affirmed as they were brought to stand on this magnificent glassy sea to strum the harps of God.
John had seen this sea before.
Early in his vision, the grandeur of God’s court had been revealed to him in all its splendor. At the center was God’s throne, brilliantly arrayed in a rainbow of light, and all around the throne of God hsd been the same flashing sea.
The Song of Moses and the Song of the Lamb
It seems this gleaming expanse was again being revealed, for now God’s beloved ones, who had prevailed through the horrors of the two beasts, were to take their place among the seraphim and elders. As John keened his ear to their heavenly music, he heard a song he recognized. It was the Song of Moses, sung in exuberant thanksgiving at the crossing of the Red Sea.
It was also the Song of the Lamb, the Redeemer. Their hearts were filled with joyous admiration, exalting Almighty God, eternal Sovereign of all that is, for God was about to engineer a similar victory over their enemies.
Who would not reverence God? they exclaimed. Who would not glorify you, the One and Only? And they answered their own questions. No one, of course! For every nation, every people group, once this was done, will have come to throne of God and thrown themselves prostrate in fear and trembling before Mighty God.
Only after John had absorbed the impact of the seven angels with their terrifying burdens, and the believers now standing before God, did he observe the centerpiece of this extraordinary and amazing sign. The Testimony, residing in the center of the true, heavenly temple.
Historicist
Acting as a bridge between chapter 14 and chapter 16, John’s oracle is intended to encourage his audience that, as unimaginably awful as things were going to get, they would not only survive, but triumph. God’s final seven judgments, held in store for the Roman Church which had so persecuted true believers, would bring this false regime to its decisive end.
The glassy sea, which before was surrounded by rainbow, now flashed with fire because of the furnace of God’s wrath preparing to purge the earth of the evils brought about by the Roman Church’s leadership. Yet, rather than show believers the suffering they too would endure as God’s wrath swept through the Roman Church, God shifted their view to the heavenly perspective.
They were instead conquerors, for they had obdurately refused to fall sway to the leaders within the Roman church and their schemes. These true believers had kept themselves pure from the Roman Church’s definitive marks, and they had never worshiped at the altars and icons manufactured at the Roman Church’s behest.

Preterist
John’s previous vision of the Sea like Glass, in chapter 4, came just before the angels blew on their seven trumpets of judgment. Once again, the Sea like Glass and its scene of glorious worship would precede the pouring forth of God’s seven bowls of judgment.
Because interpreters in this camp read John’s Revelation to be concerning the Jewish Rebellion of 66 – 70 CE, this moment is understood as the hour just before Rome’s final assault on Jerusalem. What had been God’s holy city, shining on God’s holy Mount Zion, would now be torn down, from polished granite block to lustrous marble pillar.
Those singing in the presence of God were the many martyrs who had prevailed in their words and actions, remaining faithful to God to the death, for their victory had come at the expense of their lives. The martyrs’ song had the same jubilant ring as the Hebrew captives who realized God had freed them forever from their lives of enslavement in Egypt and swept away their oppressors.
But it was more so the Song of Lamb, for Jerusalem had now become Egypt. Millennia before, Moses and Miriam had led the people to sing,
I will sing to the Lord, for he has triumphed gloriously;
Exodus 15:1 (NRSVUE, emphasis added)
horse and rider he has thrown into the sea.
In the same way God had caused the Red Sea to overwhelm the Egyptian cavalry, now a vast red sea of blood flowing from the winepress of God’s wrath would overwhelm the horses and riders of the dragon’s forces.
Then the winepress was trampled on outside of the city, and blood came out of the winepress, up to the bridles of the horses, for a thousand six hundred stadiōn.
Revelation 14:20

John’s audience would have gotten the connection immediately and understood. Their Exodus was coming soon, and though it would involve great hardship, their enemies would be defeated.and their joy would be made complete.