The Two Most Important Doctrines in the World

Ran across this really great quote in a book I was reading yesterday. I cannot seem to find its actual source though:

When asked by a student why God had allows atheism and agnosticism to exist in the world, and why, indeed, it grows, Rabbi Zusya replied:

You must know that atheism and agnosticism are two of the most important doctrines in the world. Imagine if people were absolutely sure that God existed. They would see a hungry man in the street but would not feed him. They would say, ‘Don’t worry, God will provide. He will not allow a man to go hungry.’ But since we all sometimes question the existence of God, when we see another human being in need, we all run to help, because perhaps we are the only ones to assist him.