A.T. Robertson Commentary Revelation 15:2

"And I saw as it were a sea of glass mingled with fire; and them that come off victorious from the beast, and from his image, and from the number of his name, standing by the sea of glass, having harps of God." — Revelation 15:2 (ASV)

As it were a glassy sea (ως θαλασσαν υαλινην). Accusative case after ειδον and ως here, not in 4:6, which see for the symbol.

Mingled with fire (μεμιγμενην πυρ). Perfect passive participle of μιγνυμ, to mix, and the associative instrumental case πυρ. This item not in 4:6 (a vision of peace), but here it adds to the splendour of the vision. This parenthesis (2-4) gives a picture of the martyrs in their state of bliss.

Them that come off victorious (τους νικωντας). Present active articular participle of νικαω, accusative after ειδον, "those that come off victorious" (14:4).

From the beast and from his image (εκ του θηριου κα εκ της εικονος αυτου). This use of εκ after νικαω is unusual, also with εκ του αριθμου. For these items see 13:1,14,17; 14:9,11; 19:20; 20:4.

By the glassy sea (επ την θαλασσαν την υαλινην). Or "upon" more likely (4:6) with the accusative as in Mt 14:25f..

Harps of God (κιθαρας του θεου). Objective genitive, for the worship of God (5:8; 14:2; 1 Chronicles 16:42).