I find most interesting the question of which God/religion to believe in. How do they deal with the fact that the actual, historical reason that they believe in their specific God/religion is because they were born into it (most likely - not true for everyone). Have they ever considered switching religions? What was their reason not to do so?

This usually leads to very interesting discussions on the "proofs" of their religion. And they tend to be interesting indeed.

Also, I might start the debate off by more general questions, e.g. "how do you define evidence, what do you consider knowledge to be", etc. E.g. I really want to understand how they know that their God/religion is founded on truth, and not on "alien teenagers", Matrix, etc. At least theoretically.