Set limits at the beginning of the visit about what topics are off limits, and then stick to what you say.
"Religion is a private matter and I couldn't imagine discussing that with you. As the head of the household, I've decided it's not in our best interests to converse about this in any way, no matter what your reasons are. Having said that, I hope you will respect our wishes by not bringing it up directly or indirectly."
When they do (because you know they will), use the broken record technique and repeat exactly what you said before.
The second time they do, say, "I am not sure if you remember this or not but we talked about this twice already. What can I do to help you adhere to this reasonable request of mine?"
The third time say, "This is now the third time you're bringing this up, even though I've politely asked you not to. I am wondering if you would feel more comfortable at a hotel, so you will be relieved of the pressure you seem to be feeling due to being here 24/7. What do you think?"