John Gill Commentary Psalms 99:3

"Let them praise thy great and terrible name: Holy is he." — Psalms 99:3 (ASV)

Let them praise your great and terrible name
All people, especially the Lord's people; those that dwell in Zion, where his name is great, in high esteem, venerable, and valued; as his name Jesus, or Saviour, is amiable and lovely to his saints; and his name, the Lion of the tribe of Judah, is terrible to his enemies:

for it is holy ;
his name is holy, as well as reverend and great; his nature is holy, both divine and human; holy in all his ways and works; and is holiness to his people, and therefore worthy of praise; holiness is the ground and foundation of his praise from the seraphim, (Isaiah 6:3) .