In Revelation 15, John sees a prelude to God pouring out his wrath in the seven bowl judgements of Revelation 16. The saints have cried "How Long?" long enough. Now a just God shall do what he promised. When he does, the saints break into song, singing the old song of Moses and the Lamb: "Great and amazing are your deeds, . . . O King of the nations! Who will not fear you and glorify your name?" (Rev 15:3–4). God's justice summons the saints' praise.