In the vision of Isaiah 6, the prophet saw the angels around the throne of God repeatedly declaring, “Holy, Holy, Holy, is the Lord of hosts.”
We ourselves often speak and sing of this truth. But what does it mean that God is holy?
It means that He is set apart and distinct. He is “wholly other” — there is nothing that corresponds to Him. God’s holiness is His separateness from everything unclean, profane, evil, and common. It is His moral perfection and absolute rightness and purity.
But you don’t have to think long before you realize that many things on earth are called “holy” — set apart to distinct uses for God — the ground, assemblies, Sabbaths, nation of Israel, the city Jerusalem, garments, promises, men, women, Scriptures, hands, faith, even a kiss!
If there are classes of some of these that are separate and unique from others, to be used for God, then what does it mean when we say that God is holy? To say that God is holy is to say that He is devoted to Himself. There is no higher reality to which He must conform to be holy. He is not holy because He keeps the Law; the Law is holy because He wrote it and in Himself He is the standard.
He is incomparable. His holiness is his utterly unique divine essence. It determines all that he is and does and is determined by no one. His holiness is what he is as God which no one else is or ever will be. Call it his majesty, his divinity, his greatness, his value as the pearl of great price. In the end language runs out. In the word “holy” we have sailed to the world’s end in the utter silence of reverence and wonder and awe. [John Piper, sermon on Is. 6:1-8.]