Oecumenius

Also before the throne there is a sea of glass, like crystal. The vision of the sea represents the multitude. But what about glass and crystal? The pure ones. And it is shown to be free from every stain by the holy fathers concerning God, who are as numerous as the sea (Isa. 10:22; Rom. 9:27). For Daniel says that "thousands upon thousands stood before Him, and ten thousand times ten thousand served Him." (Dan. 7:10) And being such in number, all are pure, resembling glass and crystal. …

Commentary on Revelation ↗

Irenaeus

It is not possible that the Gospels can be either more or fewer in number than they are. For, since there are four zones of the world in which we live, and four principal winds, while the Church is scattered throughout all the world, and the "pillar and ground" of the Church is the Gospel and the spirit of life; it is fitting that she should have four pillars, breathing out immortality on every side, and vivifying men afresh. …

Against Heresies Book 3 ↗

Bede

And before the throne, there was as it were a sea of glass, like crystal. This refers to the faith of true baptism, which is related to glass, in which nothing different is seen on the outside than what it bears within. The grace of baptism is also figured by crystal, which is formed from water into ice and a precious stone.

Commentary on Revelation ↗

Victorinus of Pettau

"And before the throne there was, as it were, a sea of glass like to crystal." That is the gift of baptism which He sheds forth through His Son in time of repentance, before He executes judgment. It is therefore before the throne, that is, the judgment.And when he says a sea of glass like to crystal, he shows that it is pure water, smooth, not agitated by the wind, not flowing down as on a slope, but given to be immoveable as the house of God."And round about the throne were four living creatures." The four living creatures are the …

Commentary on the Apocalypse of the Blessed John ↗