These words are nowhere to be found in the Psalter. If the Psalter is the compendium of perfect praise you would surely expect the holy angels, on such a heavenly mission as this, to sing from the Psalter. Did the holy angels set a bad example to the Shepherds? Is it just permissible for the angels to sing words not found in the Psalter but redeemed souls on earth are not?
The redeemed and glorified saints in heaven do not sing the 150 Psalms of David. The collective representatives of the redeemed in glory, in their worship of Jesus Christ, do not sing from the Psalter, Revelation 5:8-10: And when he had taken the book, the four beasts and four and twenty elders fell down before the Lamb, having every one of them harps, and golden vials full of odours, which are the prayers of saints. And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation; And hast made us unto our God kings and priests: and we shall reign on the earth.
The saints who will yet overcome the Beast do not sing from the Psalter. They are said to sing the song of Moses and the song of the Lamb, Revelation 15:2-4: And I saw as it were a sea of glass mingled with fire: and them that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, stand on the sea of glass, having the harps of God. And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, Lord God Almighty; just and true are thy ways, thou King of saints. Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.
Previous posts on this subject:
1. The terms 'Psalms, Hymns and Spiritual Songs' inEphesians 5:19 & Colossians 3:16 do not refer to the 150 Psalms of the Book of Psalms.
2. Old Testament and New Testament saints did not live by this exclusive psalmist rule.