God appears to the prophet Isaiah in a vision. Isaiah sees the Lord on His throne, high and exalted. Seraphim are singing God’s praises, calling out, “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory.”
These two lines could be the motto for the entire Old Testament. We often think of holiness as referring to purity or being sinless, but that’s only part of the meaning. The essence of the word “holy” is that God is separate, not just from sin, but from everything.
He is separate from all creation, different than all creation, transcendent over all creation. He is so far above everything and everyone that nothing can even be compared to God. He is the incomparable God. That’s what it means to say God is holy.
Holiness is not just one attribute of God. More precisely, it is the sum of all God’s attributes because God is infinite, sovereign, eternal, omniscient, and much more. He is different. He is holy. Holiness refers to the divinity of God: all that makes Him not human, but God. Holiness refers to God being separate from us, different from us, and far above us. God’s holiness is so important because it is the sum of all God’s attributes, of all that makes God who He is.
No wonder the seraphim cry out. We cry out with them. “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory!”