It seems difficult for some people to consider hypothetical situations. For religionists this difficulty is compounded by the fact that they hold dogmas that there are some ideas which, if even considered, will doom them in some way. This results in an inability for many to imagine a reality without a god.
During the course of debating with Christians I realized that many of them have probably never seriously considered the possibility that there could be no god, and are unable to imagine it. Even when confronted with someone who disbelieves they respond with "you believe, you just hate God!" At this point there is no debate.
I came up with a litmus test for theists after this. If they fail it then there is little point arguing with them. It is the following question.
If there was no god, no creator of the universe, would you want to know it?
This question demands that they consider the possibility, however briefly. It also indicates how open-minded they are if they answer. I think it's a fun question. I used it on my father when I first "came out" to him and he was fairly stymied by it.
Do you have any good show-stopper questions or statements?