Reductio ad Absurdum: How to most efficently communicate with theists

[drfuzzy]

What would be your own example of such an element.  One which could be discussed without the believer feeling (or recognizing) the threat to their beliefs?

Sorry, Rhythm!  Got busy last night and forgot to look up an example.  Ok, here is a variation of one of Boghossian's "interventions":

One amusing encounter Boghossian describes was a chat with two women in a diner.  They were somewhat miffed because a man had just spent a few minutes trying to convince them to attend his church.  

They were upset about the encounter, and told Boghossian that they already had a church.

PB: Well, why not go to his church? Then you would have TWO churches!  

They were confused.  

PB: Wouldn't two churches be better?  Two points of view? Covering all the bases? This other church might have things that your church is missing.

Women:  "Our church isn't missing anything!"

I won't write out the whole dialogue, but he gently questions how they are so certain that their church is perfect, their church is better than the other guys' church, etc.  How did they know they were "saved", when the other man was so certain that they weren't?  Why didn't they ask him for his reasons?  Etc.
PB didn't attack their belief in god.  He just got them to think about, even if it was only for a moment, why they were so sure that their Protestant church was "right" and this other guy's Protestant church was "wrong".  He got them to think about that assumption, and then he walked away.

From my own experience, it is often these small questions that lead to the larger ones.  The "waitaminnit, why DID I think that?" questions are the effective ones.  Attacking core beliefs gets a person to shut down.  Little things, though, can be fun.  That's why I like Boghossian's approach.  Not that I'm any good at it.   Yet.