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. Revelation 15:2
What a beautiful scene. But why is this the second sea of glass? The sea of glass is mentioned in Rev. 4:6 also. But this sea is mingled with fire. Not active, burning, fire, but fire embedded in the sea, as a memorial. The two mentions of the sea could be confusing.
I think of the picture of a boxer before and after his fight. He is the same man, but after the fight he is bloodied, but happy. He has won.
So this is a sea of victory. It is after the Great Tribulation.
There are those who believe the church will go through the Great Tribulation. I'll put it crudely to save time. Just as people threaten Americans with threats every day non-stop, so do unscrupulous preachers try to force this time upon the church. Here comes the crude part. There's money in threatening people!
Those waiting for the pre-tribulation rapture that the Bible promises in Rev. 3:10, are not good customers for great, earthly or worldly projects. They are wonderful missionaries and witnesses though. They offer rescue to those on a stormy sea, rather than the hardship of an over-crowded and under-supplied life boat.
I can't prove this, but could it be that those who teach that we will go through the tribulation do so to discourage further study of this wonderful book? How many churches that teach or believe we will be in the tribulation even teach the book at all? I believe their practice is to fend off further study.
Then there is the earth versus heaven angle. Some guys are much more comfortable with the here and now than with heaven, and their teaching shows it.
Nowhere in the Bible is the church warned about this great trial. It is "the time of Jacob's trouble". Jacob is Israel. We are the church. It's as simple as that.