To answer the first question "what would it take to shake the beliefs of an agnostic" I would have to reply nothing short of the coming of the messiah or god personally revealing himself. I have often thought about the question of god and it seems to me that their is no way for their to be a god. If their is a god why did he suddenly stop his interaction with the world when technology started to advance?? I would say that society wised up. If anyone tried to say they were a prophet in modern times it would be scientifically disproves, and they would have been thought of as a quack. It seems to me, that god serves the purpose of giving people a reason to live a more ethical life, and without the reason they would not be able to function as ethically which ultimately translates into efficiency.