When we say that God is holy, what does that mean to you? Pure, yes. But the definition of holiness is not exhausted by purity. When the angels around the throne of God constantly say, “Holy, Holy, Holy” (Isa. 6:3; Rev. 4:8), they aren’t just saying “Purity, Purity, Purity,” but more than that. The word “holy” means to be set apart.
When the Scriptures declare that God is holy (Psa. 99:9), it means that He is not like any other. There is no one like Him. He is matchless and incomparable. No one is His equal. He is unique in Himself and His Person, unsurpassed in beauty, worth, and perfection. He is set apart unto Himself. As it’s been said, “God is always the greatest thing in existence.”
When the Bible refers to “the holy Scriptures” (2 Tim. 3:15), “holy” means much the same thing. The Bible is not like any other thing. There is no other book like it. It has no equal. It is incomparable, unsurpassed in its beauty, worth, and perfection. The Bible is set apart from all other books. It is the Word of God. source