This is a continuation of verse 3, the song of Moses and the song of the Lamb.
"Who will not FEAR, O LORD, and GLORIFY Thy name?
This is the most likely response of a true believer who realizes the awesomeness of God and how to FEAR this almighty God. The result is a spontaneous GLORIFYING His Name!
In the Old Testament, Jeremiah came to the same conclusion (Jeremiah 10:6-10). So this theme is an eternal theme, and we, present day believers, would do very well to understand this question.
This question parallels the one posed in Revelation 13:4 about the beast. But remember who is singing this time (verse 2).
The main word to emphasis here is "ALONE". This HOLINESS is unique to God and the Lamb. The four living creatures understood this (Revelation 4:8). At some time, everyone will understand this (Philippians 2:9-11).
According to Watchman Nee, HOLY is a description of God's nature.
The individualism implied above is now expanded to 'nations'. That nations, all of them, will come and WORSHIP before the Lamb has been prophesied from old (Psalm 66:1-4, Psalm 72:8-11, Psalm 86:8-10, Isaiah 66:22-24, and Zechariah 14:16-21).
Looking into the future, even in the new earth and new heaven, this principle exists (Revelation 21:22-24-27).
The RIGHTEOUS ACTS are God's works according to Watchman Nee. This is HOLINESS (nature) revealed in works.
It is implied that the RIGHTEOUS ACTS have been revealed to the singers, both individually and nationally. These RIGHTEOUS ACTS expose the evil of the last days. That evil penetrates governments should not be a surprise (see comments in Revelation 12:3).
Today, we often hear, "That isn't FAIR!" But by this time, the coming judgments embodying the FULL/COMPLETE WRATH of God (verse 7) will be absolutely FAIR because of God's HOLINESS and RIGHTEOUS ACTS!