Rev. 15:3 – They sang the song of God’s servant Moses and the song of the Lamb: Great and awe-inspiring are Your works, Lord God the Almighty; righteous and true are Your ways, King of the Nations. 4 Lord, who will not fear and glorify Your name? Because You alone are holy, for all the nations will come and worship before You because Your righteous acts have been revealed. (HCSB)
They sang the song
Those who have won the victory over the beast, his image, and the number of his name now sing the song of God’s servant Moses and the song of the Lamb (vv. 3-4). It appears these are two songs with a common theme. They show the unity of the Old Covenant and the New Covenant in redemption. The song of Moses alludes to Ex. 15:1-19, where Moses thanks God for deliverance from the Egyptians at the Red Sea. However, it’s possible that John has Deuteronomy 32 in mind because the first phrase – “Great and awe-inspiring are Your works, Lord God, the Almighty; righteous and true are Your ways, King of the Nations” – may be drawn from Deut. 32:3-4.
The song of the Lamb may be what John hears in Rev. 5:9:
You are worthy to take the scroll
and to open its seals;
because You were slaughtered,
and You redeemed [people] for God by Your blood
from every tribe and language and people and nation.
You made them a kingdom and priests to our God,
and they will reign on the earth.