A little help please …

Roger Ebert says in his review of the Coen Brothers’ movie, A Serious Man,

I’m sure you’ve heard the old joke where Job asks the Lord why everything in his life is going wrong. Remember what the Lord replies? If you don’t remember the joke, ask anyone. I can’t prove it but I’m absolutely certain more than half of everyone on Earth has heard some version of that joke.

I am of the other half of the world. Can someone tell me the punch line?