Who are the people standing on the sea of glass in Revelation 15:2?
They are the martyred Tribulation saints who overcame the beast, his image, his mark, and the number of his name. Having been victorious over the Antichrist and Satan indwelling him, they are already in the Lord's presence in glory, standing on the sea of glass and holding the harps of God.
Les explains that Revelation 15:2 gives a vision in heaven of a sea of glass mingled with fire. Those standing on it are the martyred Tribulation saints who have gotten the victory over the beast, or the Antichrist, and over Satan who is indwelling him, as well as over his image, his mark, and the number of his name. They are already in the Lord's presence in glory, having the harps of God. Les notes this does not mean heaven is only harp-strumming. They then sing the song of Moses and the song of the Lamb, which are two separate songs; the song of Moses points back to Exodus 15 after Israel came through the Red Sea.
And I saw as it were a sea of glass mingled with fire: and them that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, stand on the sea of glass, having the harps of God.