Revelation 15

And I saw something like a sea of glass mingled with fire, and those who have the victory over the beast, over his image and over his mark and over the number of his name, standing on the sea of glass, having harps of God. They sing the song of Moses, the servant of God, and the song of the Lamb, vv.2-3

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The seven last plagues of God are about to be complete. Seven angels have seven bowls of wrath. But before this happens John sees this beautiful scene. He sees a sea of glass mingled with fire. Back in chapter four John had already seen this sea, but this time it's mingled with fire. This doesn't mean a sea full of glass but the sea-bed is made of glass that the water is in; probably more like a glass bowl. And on this sea John sees the overcomers of the beast standing on it with harps of God. These overcomers possibly are the 144,000 for they are about to sing the song of Moses and we saw in the last chapter that they sung before the throne of God, though a different song. In Deuteronomy 32 we see a song of Moses, yet this is not the same song for this will be a song after the judgments of God are manifest (v.4). Interesting that they're singing about judgments and it's not a sad song they're singing but one of praise to the Lord. But it is God's judgements that set things right; we see David crave God's judgments like honey (Ps. 19:9-10). When justice is upheld there is peace for wrongs are made right. May all bring praise to the One who is the source of justice.