Revelation 15:2

And those who have conquered the beast and its image, etc. Those who conquer the beast's deceits appear to stand by the fiery baptism, striving, according to the Apostle, to contend for the faith once delivered to the saints (Jude 3).

Having the harps of God, and singing, etc. That is, bearing hearts dedicated to praising God, and singing with the truth of both Testaments. Or, the flesh extended on the wood of the passion. Where not only the sound of the voice, but also the …

And I saw something like a sea of glass mingled with fire. That is, the translucent fountain of baptism, consecrated with the fire of the Holy Spirit. Or also, in regard to the quality of fire, reddened by martyrdom.

The song was sung while the angels were coming out, with their plagues, who are therefore mentioned both before and after it, verses 1-6. chap. xv, 1-6, And I saw as it were a sea of glass mingled with fire - It was before "clear as crystal," chap. iv, 6, but now mingled with fire, which devours the adversaries.

And I saw, as it were, a sea of glass mingled with fire,....

It has a pregnant sense: "standing" implies rest, Greek "epi" with the accusative case implies motion "towards." Thus the meaning is, Having come TO the sea, and now standing AT it. In Mt 14:26, where Christ walks on the sea, the Greek oldest manuscripts have the genitive, not the accusative as here.

And I saw as it were a sea of glass - In Rev 4:6, a similar vision is recorded - “And before the throne there was a sea of glass, like unto crystal.” See the notes on that passage. The sea of glass here means a sea clear, pellucid, like glass: an expanse that seemed to be made of glass. There it was entirely clear; here it is mingled with fire. Mingled with fire - That is, a portion of the sea was red like fire. It was not all clear and pellucid, as in Rev 4:6, but it was as it were a tesselated expanse, composed in part of what seemed to be glass, and in part of a material of a red or fiery color. In the former case Rev 4:6, the emblem was designed to represent the pure worship of heaven without reference to any other symbolic design, and hence, the sea is wholly clear and pellucid; here, in connection with the purpose of furnishing an appropriate symbol of the divine majesty, there is united the idea of punishment on the foes of God, represented by the fiery or red color. If it is proper, from conjecture, to suggest the meaning of this as an emblem, it would be that the foundation - the main element - of all the divine dealings is justice or holiness - represented by the portion of the sea that seemed to be glass; and that there was, in this case, intermingled with that, the image of wrath or anger - represented by the portion that was fiery or red. The very sight of the pavement, therefore, on which they stood when worshipping God, would keep before their minds impressive views of his character and dealings. And them that had gotten the victory over the beast - Rev 13:11. That is, they who had gained a victory in times of persecution and temptation; or they whom the “beast” had not been able, by arts or arms, to subdue. […]