"For I am the Lord your God. You must consecrate yourselves and be holy, because I am holy." (Leviticus 11:44)
"Exalt the Lord our God, and worship at his holy mountain in Jerusalem, for the Lord our God is holy." (Psalm 99:9)
Speaking of the seraphim (angels) which he observed in a vision of God's throne, Isaiah wrote: "They were calling out to each other, 'Holy, holy, holy is the Lord of Heaven's Armies! The whole earth is filled with his glory!'" (Isaiah 6:3)
In a similar vision of God's throne, John wrote that the creatures around God's throne chanted day and night: "Holy, holy, holy is the Lord God Almighty - the one who always was, who is, and who is still to come." (Revelation 4:8)
What does it mean to be holy?
Strong's Exhaustive Concordance informs us that the Hebrew word for "holy" is "qadowsh." Gesenius' Hebrew-Chaldee Lexicon further informs us that the word is indicative of that which is pure, and free from the defilement of crimes/sins and other impure or profane things.
This is consistent with what David said about God. He wrote: "The Lord is righteous in everything he does; he is filled with kindness." (Psalm 145:17) An unidentified psalmist wrote about God: "Your righteousness, O God, reaches to the highest heavens." (Psalm 71:19) And: "Everything he does reveals his glory and majesty. His righteousness never fails." (Psalm 111:3) Likewise, James wrote that "God cannot be tempted with evil." (James 1:13) The author of Hebrews said that it is impossible for God to lie or break a promise. (Hebrews 6:18) We read in Proverbs that every word of God is pure. (Proverbs 30:5)
God is pure, clean and righteous. God is holy!