Revelation 15:2

2  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.

Those who had gotten victory over the Beast stand on what looks to John like a sea of glass. The sea of glass John sees is actually a sea of people. This is a picture of the Church made perfect resting at the throne of God. The fire seen within this sea is the Holy Spirit that dwells in their midst. Those now standing on the foundation made pure are the saints who died during the great tribulation. They obtained victory over the Beast by dying for their faith in Jesus. They proved this faith by not bowing to the image of the Beast or receiving his mark or his number. Now that the tribulation saints are in heaven, the bride is completed.