Then one of the seraphim flew to me with a live coal in his hand, which he had taken with tongs from the altar. With it he touched my mouth and said, “See this has touched your lips; your guilt is taken away and your sin atoned for. Then I heard the voice of the Lord saying: “Whom shall I send? And who will go for us?” And I said, “Here am I! Send me!  He said, “Go and tell this people: "Be ever hearing, but never understanding; be ever seeing, but never perceiving.' Make the heart of this people calloused; make their ears dull and close their eyes. Otherwise they might see with their eyes, hear with their ears, understand with their hearts, and turn and be healed.”
Isaiah 6: 6-10 (New International Version)