“There is no reason why someone who is in doubt about the existence of God should not pray for help and guidance on this topic as in other matters. Some find something comic in the idea of an agnostic praying to a God whose existence he doubts. It is surely no more unreasonable than the act of a man adrift in the ocean, trapped if a cave, or stranded on a mountainside, who cries for help though he may never be heard or fires a signal which may never be seen.”