The Catechism in Questions & Answers

54. What does it mean to say: “God is the Holy One”?

Holiness is part of God’s nature, being, and activity. Holiness incorporates majesty, inviolability, and separateness from the profane. God’s word and will are likewise holy.
The holiness of God sanctifies the place where He reveals Himself.

“Holy, holy, holy is the Lord of hosts; the whole earth is full of His glory!” Isaiah 6: 3