A.T. Robertson Commentary Revelation 15:3

"And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, O Lord God, the Almighty; righteous and true are thy ways, thou King of the ages." — Revelation 15:3 (ASV)

The song of Moses (την ωιδην του Μωυσεως). Exodus 14:31; 15:1-19. A song of victory like that of Moses after crossing the Red Sea.

And the song of the Lamb (την ωιδην του αρνιου). A separate note of victory like that of Moses, though one song, not two. Charles finds it impossible to reconcile the two expressions, if genuine, but it is a needless objection. The words come from the O.T.: "great" (μεγαλα) from Ps 111:2, "wonderful" (θαυμαστα) from Ps 139:14, "O Lord God the Almighty" (Κυριε ο θεος ο παντοκρατωρ) from Am 4:13 (Revelation 4:8), "righteous and true" (δικαια κα αληθινα) from De 32:4, "Thou King of the ages" (ο βασιλευς των αιωνων) like Jer 10:10; 1 Timothy 1:17. Some MSS. have "the king of the saints" and some "the king of the nations," like Jer 10:7. John thus combines in Hebraic tone the expressions of the old and the new in the song to the Glorified Messiah.