2And 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.

"a sea of ​​glass mixed with fire", which represents, possibly, the terrible persecution that the Beast will submit to Christians during the period of the Great Tribulation. During this period no man will be able to buy or sell, unless he has engraved in his hand or on his forehead the Mark of the Beast, the number 666.
These days will be very difficult, in which no Christian will survive unless it has been sealed with the Mark of God.

This verse tells us that they will not only endure that period, but that they have achieved victory over the beast and its image and its mark and the number of its name. Here we have the saints of the Tribulation, who have passed through the fire of persecution and yet their song has not ceased. They have the harps of God