What is the sea of glass mingled with fire?
The sea of glass mingled with fire is the place in Heaven where the raptured Church first stands after the tribulation, and it represents the fire that will test the value of every believer's works and ministry.
Revelation 15:2 says, "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." After enduring much persecution, the Church is raptured and finds itself in Heaven, and the very first place it arrives is before this sea of glass mingled with fire. This is understood to be the fire that will try the works each believer has done, such as the souls brought into the Kingdom, the ministry that edified others, and the lives that were delivered from captivity through obedience.
This judgment reveals whether the good works done in ministry truly bore fruit that helped bring others to this sea of glass. Only those whose labor has borne fruit will stand there; those without fruit will not appear in this judgment at all, because that judgment for fruitlessness is determined while still on the earth, and the fruitless are not raptured.
1 Corinthians 3:6-15 is used to explain this fire in detail. Paul planted and Apollos watered, showing that both those who bring souls into the Kingdom and those who nurture them along the way will be rewarded for their own labor. Verse 12 speaks of building on the foundation of Christ with gold, silver, precious stones, wood, hay, or stubble, and verse 13 declares, "Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is." The day the Church is taken to Heaven is the day these works will be manifest at the sea of glass.
Verses 14 and 15 explain the outcome: "If any man's work abide which he hath built thereupon, he shall receive a reward. If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire." So some believers who led others into the Kingdom may see those they nurtured backslide or depart from the faith, and in such cases they will suffer loss and receive fewer rewards, but their own salvation is never in danger. The sea of glass mingled with fire, then, stands as the scene of this judgment of good works, distinct from the judgment of the wicked, and it precedes the giving out of the rewards of the saints described afterward, such as new bodies, ruling and reigning, and eternal glory.
The scriptures
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.
Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.
I have planted, Apollos watered; but God gave the increase. So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase. Now he that planteth and he that watereth are one: and every man shall receive his own reward according to his own labour. For we are labourers together with God: ye are God’s husbandry, ye are God’s building. According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon. For other foundation can no man lay than that is laid, which is Jesus Christ. Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble; Every man’s work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man’s work of what sort it is. If any man’s work abide which he hath built thereupon, he shall receive a reward. If any man’s work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
And the nations were angry, and thy wrath is come, and the time of the dead, that they should be judged, and that thou shouldest give reward unto thy servants the prophets, and to the saints, and them that fear thy name, small and great; and shouldest destroy them which destroy the earth.
If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you. Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples.
Ye have not chosen me, but I have chosen you, and ordained you, that ye should go and bring forth fruit, and that your fruit should remain: that whatsoever ye shall ask of the Father in my name, he may give it you.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.