To me this is also an age and maturity thing. When I was younger I was too worried to be perceived as nice to not say something when an adult really crossed the line into my business. Now, fuck that. Nice girls need to say, back off!!
You dont have to explain what you believe or dont believe, which was what OP was scared to do. She isnt the type who wants to discuss her private views.
OP just needs to firmly say, this is not up for discussion and it is my business.