The term Holy One of Israel often appears in Isaiah as a title for the Lord. God is:
the righteous and just King of heaven and earth (Isaiah 6:3–5),
the incomparable God, meaning no one is like him (Isaiah 40:25), and
the Redeemer (the one who rescues and saves) of his people (Isaiah 43:14).
Because of this, it was a serious and foolish sin for Judah to reject him.