‘And they sing the song of Moses, the servant of God, and the song of the Lamb, saying, “Great and marvellous are your works (Psalms 104:24; Psalms 111:2; Psalms 139:14), Oh Lord God, the Almighty. Righteous and true are your ways (Deuteronomy 32:4; Psalms 145:17), you King of the Ages (Jeremiah 10:10). Who shall not fear, Oh Lord, and glorify your name (Psalms 86:9)? For you only are holy (Psalms 86:10; Psalms 99:3; Psalms 99:5; Psalms 99:9). For all the nations shall come and worship before you (Psalms 86:9-12), for your righteous acts have been fully revealed”.'
There are two songs here combined in one, the song of Moses and the song of the Lamb. The song declares that just as the sign depicting the seven plagues (v. 1) was great and marvellous, so are God's works great and marvellous (v. 4). Thus the works include His works of judgment. Here, after they have taken place, they have the approval of God's redeemed people.
The song of Moses (Deuteronomy 31:30) is described in Deuteronomy 32. It is a song celebrating the One Who is a God of faithfulness, just and right. His work is perfect and His ways are justice (v. 4) in spite of His people's unfaithfulness and failure (v. 5). But then, after a catalogue of their failure, it declares that He kills, but He makes alive. He wounds, but He heals (v. 39) and He avenges the blood of His servants and makes expiation for His people (v. 43). This deliverance is what God indeed has wrought, as witness those gathered here, and this judgment is what He is about to carry out. Thus this song epitomises the song of Moses.
(The song in Exodus 15 is nowhere called the song of Moses. That was the song of redeemed Israel).
The fact that it is also called the song of the Lamb shows that these words refer to Him. He is the Lord God, the Almighty, Whose ways are marvellous, true and righteous. He is the One to be feared and glorified. He is the Holy One before Whom the nations will worship (Revelation 5:9). He is the One Whose mighty acts have been revealed, in, for example, the opening of the seals and the cleansing of His people. He is the One Who is great and marvellous, the King of the Ages, the Eternal King.