The idea of the holiness of God is important in the preaching of Isaiah. When Isaiah received his call to prophesy he was overwhelmed with the understanding which came to him of the absolute moral purity and holiness of God, contrasted with which the unholiness and evil of the people who worshipped him was a terrible thing (Isaiah 6). Not until Isaiah had received purification by fire in the vision which accompanied his call, could he speak for the holy God. God's holiness judges all that is unholy and so Isaiah condemns the evil that he sees in the lives of the people of Jerusalem (1: 4, 5: 19, 5: 24), because they are not fit to worship the holy God. Isaiah's vision also reveals the mighty power of God to him; God is the Mighty One, the Only God, as well as the Holy One.