Isaiah 6:2-3: "2 Above him [the Lord] stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. 3 And one called to another and said: 'Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory!'"
Revelation 4:2-11: "2 At once I was in the Spirit, and behold, a throne stood in heaven, with one seated on the throne. 3 And he who sat there had the appearance of jasper and carnelian, and around the throne was a rainbow that had the appearance of an emerald. 4 Around the throne were twenty-four thrones, and seated on the thrones were twenty-four elders, clothed in white garments, with golden crowns on their heads. 5 From the throne came flashes of lightning, and rumblings and peals of thunder, and before the throne were burning seven torches of fire, which are the seven spirits of God, 6 and before the throne there was as it were a sea of glass, like crystal.
"And around the throne, on each side of the throne, are four living creatures, full of eyes in front and behind: 7 the first living creature like a lion, the second living creature like an ox, the third living creature with the face of a man, and the fourth living creature like an eagle in flight. 8 And the four living creatures, each of them with six wings, are full of eyes all around and within, and day and night they never cease to say, 'Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!' 9 And whenever the living creatures give glory and honor and thanks to him who is seated on the throne, who lives forever and ever, 10 the twenty-four elders fall down before him who is seated on the throne and worship him who lives forever and ever. They cast their crowns before the throne, saying, 11 'Worthy are you, our Lord and God, to receive glory and honor and power, for you created all things, and by your will they existed and were created.'"
Exodus 15:11: "'11 Who is like you, O LORD, among the gods? Who is like you, majestic in holiness, awesome in glorious deeds, doing wonders?'"
Revelation 7:9-12: "9 After this I looked, and behold, a great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands, 10 and crying out with a loud voice, 'Salvation belongs to our God who sits on the throne, and to the Lamb!' 11 And all the angels were standing around the throne and around the elders and the four living creatures, and they fell on their faces before the throne and worshiped God, 12 saying, 'Amen! Blessing and glory and wisdom and thanksgiving and honor and power and might be to our God forever and ever! Amen.'"
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The text is public domain:
Holy, holy, holy! Lord God Almighty!
Early in the morning our song shall rise to Thee;
Holy, holy, holy, merciful and mighty!
God in three persons, blessed Trinity!
Holy, holy, holy! All the saints adore Thee,
Casting down their golden crowns around the glassy sea;
Cherubim and seraphim falling down before Thee,
Which wert and art and evermore shalt be.
Holy, holy, holy! Though the darkness hide Thee,
Though the eye of sinful man Thy glory may not see,
Only Thou art holy; there is none beside Thee,
Perfect in pow'r, in love, and purity.
Holy, holy, holy! Lord God Almighty!
All Thy works shall praise Thy name in earth and sky and sea.
Holy, holy, holy, merciful and mighty!
God in three persons, blessed Trinity!
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The repeated "Holy, holy, holy!" comes from Isaiah 6:3 (although it also appears later in Revelation 4:8).
The second verse comes mostly from Revelation 4:2-11, specifically drawing on the "sea of glass" (verse 6), the elders' casting down their crowns (verse 10), and the description of God as "who was and is and is to come" (verse 8). The angels "falling down before Thee" come from Revelation 7:11: "all the angels... fell on their faces before the throne and worshiped God."
The second half of the third verse seems to come from Exodus 15:11. While the verse from Exodus asks "Who is like you?" rhetorically, the hymn verse provides an answer: "Only Thou art holy; there is none beside Thee."
"All Thy works shall praise Thy name" in the fourth verse also seems to come from Revelation 7, specifically the "great multitude that no one could number, from every nation, from all tribes and peoples and languages."
