The text's theme is drawn from John's vision of the saints in glory singing one of their great doxologies (Rev. 7:9-17). The initial reference to the "sea of crystal" is from Revelation 4:6–"Also before the throne there was what looked like a sea of glass, clear as crystal." And the final (rhetorical) reference to the "King of kings" (st. 3) comes from Revelation 17:14.
“With the whole creation we join the song: ‘Worthy is the Lamb, who was slain, to receive power and wealth and wisdom and strength and honor and glory and praise!’” (Our World Belongs to God, paragraph 58)