I don't presume to speak for smart Agnostics, but I think what you did was spot on!

I usually have a problem with "foot in mouth" disorder. I might have told her how strongly I felt that organized religions have caused more conflicts, deaths and wars than any other reason in human history. Then I would have walked to the snack table. Of course this is hind sight. Who knows what I might have really done?

I am pretty simple.. I'll pull out my phone and occupy myself while they ramble on, or if no phone I'll look around at others with an eyebrow raised and a "are you kidding me?" Look on my face. IF by chance they try to call me out, I'll start asking questions, polite at first but pending their replies I can bring out the tough questions & comments.

Seems to me like she was looking for an argument!

Just one more thing.
You could have farted (loudly) as you walked over to the snack table to punctuate your visible "statement".

It is unfortunate that a religious person has to make it all about her religion and not about respecting each person's individual goals with conflict resolution.

Sounds like you handled it fine.

You could have said: "No thanks, I'm not sure that I can apply what we've learned today on what you just brought up that quickly." ?

Well, it's an attendee not an organizer, so I think a trip to the snack bar was in order.