Albert Barnes Commentary Psalms 99:9

"Exalt ye Jehovah our God, And worship at his holy hill; For Jehovah our God is holy." — Psalms 99:9 (ASV)

Exalt the Lord our God – See the notes at (Psalms 99:5).

And worship at his holy hill – In (Psalms 99:5), this is, at his footstool. The “holy hill” refers to Zion, as the seat of the national worship.

For the Lord our God is holy – See (Psalms 99:5). This appropriately closes the psalm, by a distinct and solemn statement that the fact that Yahweh is a holy God is a reason for worshipping him. This is at all times the highest reason for adoration and praise.