My father used to say to us that stupidity is the greatest sin. He did not mean simplicity of mind, but spiritual dullness: having a dead conscience and not listening with one's heart to God.
Very few people today have any idea of the riches of the human heart. Our hearts are created to experience great things; most of us have no idea of what could happen in our lives if we would overcome our stupidity and dullness.

Excerpted from Discipleship by J. Heinrich Arnold.