This question is praise. The singers have just called God's works "great and marvellous" and His ways "just and true" (Revelation 15:3). Having seen that, they cannot imagine anyone refusing Him honor. Barnes explains "fear" here as "Reverence and adore thee," and "glorify thy name" as honoring God Himself, "the name being put for the person who bare it." The question carries its own reasons in three "for" clauses: God alone is holy, all nations will come to worship, and His judgments are now plain to see. Matthew Henry says the singers "call upon all nations to render unto God the fear, glory, and worship, due to such a discovery of his truth and justice."

The song answers its own question at once: "for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest." The expected answer is that no one should fail to fear Him. Then John sees the temple in heaven opened, and the seven angels come out with the seven plagues (Revelation 15:5-6). "The temple was filled with smoke from the glory of God, and from his power," and no one could enter "till the seven plagues of the seven angels were fulfilled" (Revelation 15:8).

Barnes links "the song of Moses" to the song Israel sang after God brought them out of Egypt (Exodus 15). Gill agrees it is the song of Exodus 15:1, not the song of Deuteronomy 32. Gill thinks this verse and the end of Revelation 15:3 seem to be taken from Jeremiah 10:7: "Who would not fear thee, O King of nations?" He notes that some early copies and versions of Revelation 15:3 read "King of nations" (as in Jeremiah 10:7) and others "King of ages," where the KJV has "King of saints." The ESV follows the reading "King of the nations."

Two Greek words are worth knowing. "Holy" here is hosios, holy in the sense of deserving reverence. "Judgments" is dikaiomata, righteous acts or righteous decrees, which is why the ESV says "your righteous acts have been revealed."

Two chapters earlier, the world asked, "Who is like unto the beast?" (Revelation 13:4). Now the people who refused the beast ask a better question: who could keep from fearing the Lord? Their song reveals God's character. Barnes says His holiness shows Him to be "a God hating sin, and loving righteousness and truth." His judgments are not hidden or random. They are "made manifest," and seeing them leads to worship.

Commentators read the promise that "all nations shall come" within their own frameworks. Barnes ties it to the fall of the anti-Christian power: "Everywhere in the New Testament the destruction of that power is connected with the promise of the speedy conversion of the world." Gill expects a time when "the fear of the Lord will be in all places, and in all men, both Jews and Gentiles," pointing to Hosea 3:5. Whatever view one takes of the timing, the song is clear: God alone is holy, and in the end the nations will worship Him.