Can you deal with a devout religious person without thinking in the back of your mind, "I can't take anything you say seriously"?
It is for times like those that I save an Alka Seltzer in my pocket. I take it out without being seen and put it in my mouth. I then crush it with my teeth and take a small drink of water. I can then foam at the mouth for a good 60 seconds while I start speaking in tongues. They generally shut up at that point!
I do not keep it in the back of my mind. I say my opinions out loud but politely and open to responses.