John Piper, from his sermon Holy, holy, holy is the Lord of hosts:
Sixth, God is holy. “And one called to another, ‘Holy, holy, holy is the Lord of hosts!’ Remember how Reepicheep, the gallant mouse, at the end of The Voyage of the Dawn Treader sailed to the end of the world in his little coracle? Well, the word “holy” is the little boat in which we reach the world’s end in the ocean of language. The possibilities of language to carry the meaning of God eventually run out and spill over the edge of the world into a vast unknown. “Holiness” carries us to the brink, and from there on the experience of God is beyond words.