I'm making up a new rule, and naming it after myself.
The Diogenes The Cynic rule posits that in order to hold a belief non-dogmatically, and be able to discuss it rationally, a person must be able to articulate under what conditions they would abandon their belief.
I think using this nifty little tool could weed out a lot of people who can't be reasoned with.
But just for fun, apply it to your own most closely held beliefs, and tell us what would make you question them.
Example: I sincerely believe in the truth and validity of the Bible, but would question this belief if archeologists found a series of closely related stories that predated the Bible that talked about the same events, effectively showing an evolution to the Bible canon.