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, Lord God Almighty; just and true are thy ways, thou King of saints.
He expresses here more fully the purpose and goal of the elect, who say from the heart, “Put the way of iniquity far from me, O Lord, and graciously teach me your law,” or, “For the sake of your law I have endured for you, O Lord,” and things similar to these. [By this scene] he wishes to make clear that these are in the number of the faithful in that they are participants in the victory mentioned. In Moses the Old Testament is signified as the New Testament is signified in the Lamb, and by this it teaches us that the elect were all equipped from both and perfected in both.… By a harmonious truth the pages of both Testaments testify to us concerning [their song], for “all the paths of the Lord are mercy and truth.” He is more aptly called the king of the nations, because he was a God not only known among the Jews, nor did the dew from heaven moisten only one fleece. But as Malachi said, “From the rising of the sun to its setting my name is great among the nations, and in every place a pure offering is sacrificed and offered to my name.”
From the song of Moses we learn the hymn sent up to God by those who were justified by the law before [the coming of] grace. From the song of the Lamb we learn the thanksgiving of those who lived in holiness after the coming of the Christ. This thanksgiving is for those benefactions and gifts from him that have come to our race, since through the holy apostles he has called all nations to the knowledge of him.
Great and marvelous are your works, Lord God Almighty, etc. This song is found in both Testaments, where the Lord, both true and merciful, is sung by all ages as the judge to be worshipped. He repeats what he had proposed, saying:
The harps, as has often been said, signify the sweet-sounding hymn of the saints to God.
And he says, let us sing that song of Moses, the one which he sang when the host of Pharaoh was drowned, "let us sing to the Lord, for he has been gloriously glorified; he cast horse and rider into the sea; he became to me a helper and a cover for salvation." (Ex. 15:1-2) And the song is a victory-song concerning the punishment of the ungodly and the victory against the Devil and the son of lawlessness, the Antichrist. And he calls the song of the Lamb that which is fitting, namely toward the Lord and toward his righteous judgment against the ungodly; therefore they admire the truth and the righteousness of the Lord.
[AD 560] Primasius of Hadrumetum on Revelation 15:3-4