The symbol of a sea of glass mingled with fire in Revelation
means, I believe, that we eventually will see crystal clearly with no such obscurity or ambiguity, that God's judgment is absolutely infallible and righteous.
Everything under Gods judgement by then the will be transparently clear
as crystal glass. The saved stand upon the shore of this glassy sea just as the rescued Israelites stood safely on the shore of the Red Sea after it had totally swallowed the pursuing armies of pharoah.
Think of this when you read about the saved standing upon a sea of glass mingled with fire, which is the lake of fire of eternal judgment.
" And I saw as it were a glassy sea mingled with fire and those who come away victorious from the beast [the Antichrist] and from his image and from the number of his name standing on the glassy sea, having harps of God.
And they sing the song of Moses, the slave of God, and the song of the Lamb, saying,
Great and wonderful are your works, Lord God the Almighty! Righteous and true are Your ways, O King of the nations!
Who will not fear, O Lord, and glorify Your name? You alone are holy; for all the nations will come and worship before You, for Your righteous judgments have been manifested." (Rev. 15:2-4)