The Lord is king; let the peoples tremble!
He sits enthroned upon the cherubim; let the earth quake!
The Lord is great in Zion;
he is exalted over all the peoples.
Let them praise your great and awesome name.
Holy is he!
Mighty King, lover of justice,
you have established equity;
you have executed justice
and righteousness in Jacob.
Extol the Lord our God;
worship at his footstool.
Holy is he!
(Lectionary, New Revised Standard version)
Today, if asked, we can struggle to describe God, and it was the same centuries ago for the Israelites. The ultimate authority for them would have been a powerful king or emperor, which is how Psalm 99 sees God. Not only that, but He is a reigning King, not a long dead king. Psalms 93 and 97 also start in the same way: 'The Lord is king'.
But today's psalm, Psalm 99, also speaks of God as holy, a word that really means set apart. Usually we say something is holy when it is set apart for a special purpose. A priest is set apart for the work of God, while a paten and chalice are set apart to use in the service of Holy Communion (ie the Eucharist or Mass). The word holy is used by the psalmist to describe the distance between God and us. He is not just perfect, but He is also not a created creature. We can say God is separated from all that is unjust, untrue, or evil in the universe. Therefore we cannot measure Him by human measures for He is divine and we are mortal.
Isaiah was to emphasise God's perfect wisdom and perfect love, and the reason why as our eternal King we should praise and worship Him, by his triple use of the word:
King of kings and Lord of lords,
like the men and women of old
we laud and praise You.
As our Holy King You sent Your Son
to rescue us from the slavery of sin,
and bring us back to You.
All too often we fail You,
but still You call us back.
May we learn to be faithful
most Holy King.
Amen.
Here is a much more detailed explanation of Holy, but I suggest only those with time, or who are really keen, pursue it:
You might just prefer to listen to this traditional hymn by First Dallas Baptist Choir: