Do you live with her? if not, have the conversation in your own home. And then say (calmly, firmly, the voice of someone who might be her daughter but who is also a confident, educated, self supporting adult) "don't you contradict me, not in my own home, not when you have no idea what you are talking about. Your friend from 20 (30, 40) years ago was either wrong or your memory has failed. Either way you can have a polite adult conversation about this or you can go home."
Then give her a moment to absorb that, because I'm thinking if you have the kind of relationship where she still says "don't contradict me" she's not used to you asserting yourself as an adult.
Then have some proof - some religious texts, pamphlets, an official website, etc - handy and say "here, you can learn more from this if you are interested. But really I'd like to talk to you a moment about religion X."
And really you will need to tell her you have converted at some point. You aren't going to tell her its your new religion fine, but then stop talking about it at all. Talking about it abstractly is just passive aggressive, the topic clearly pushes a button for her.