The holiness of God is his glory and crown. It is the blessedness of his nature. It renders him glorious in himself, and glorious to his creatures. “Holy” is more fixed as an epithet to his name than any other. This is his greatest title of honor. He is pure and unmixed light, free from all blemish in his essence, nature, and operations. He cannot be deformed by any evil. The notion of God cannot be entertained without separating from him whatever is impure and staining. Though he is majestic, eternal, almighty, wise, immutable, merciful, and whatsoever other prefections may dignify so sovereign a being, yet if we conceive him destitute of this excellent perfection, and imagine him possessed with the least contagion of evil, we make him but an infinite monster, and sully all those perfections we ascribed to him before.
It is a contradiction for him to be God and to have any darkness mixed with his light. To deny his purity, makes him no God. He that says God is not holy, speaks much worse than if he said there is no God at all. Where do we read of the angels crying out Eternal or Faithful Lord God of hosts? But we do hear them singing Holy, Holy, Holy. God swears by his holiness (Psa. 89:35). His holiness is a pledge for the assurance of his promises. Power is his hand, omniscience his eye, mercy his heart, eternity his duration, but holiness his beauty. It renders him lovely and gives beauty to all his attributes. Every action of his is free from all hints of evil. Holiness is the crown of all his attributes, the life of all his decrees, and the brightness of all his actions. Nothing is decreed by him and nothing is acted by him that is not consistent with the beauty of his holiness.
Stephen Charnock, The Existence and Attributes of God. Quoted from Voices from the Past.