If your mother is truly narcissistic then that is a different beast altogether than just parents that are unhappy that you are no longer in the cult. Narcissists feel extreme entitlement to cross boundaries.
I had to choose my boundaries carefully and then when my mother crossed them tell her clearly that she had no right to do that and immediately hang up the phone.
It's pure behaviorism, not unlike training your dog. She crosses a boundary, she immediately loses access to you. She finally learns that to get what she wants, which is to talk to you, she has to behave. No behaving, immediate loss of contact. Every time.
But, choose your boundaries first. If it's not talking about religion, when she brings it up tell her that you're no longer going to discuss religion with her. If she contines (and narcissists will because they feel entitled) hang up the phone. Ignore the immediate call back.
Then, after a length of time you are willing to accept contact from her (whatever boundary you decide, for example, weekly) answer the phone. Don't talk about the previous phone call. Continue talking as long as you want, but if she brings up religion again, hang up immediately.
Decide your boundary regarding your kids, too. If you don't mind her talking to them, but you don't want her talking religion to them, set up skype sessions where you must always be there so she doesn't have unrestricted access to your kids. If she talks religion to them, end the skype session.
These toxic personalities just don't take the hint. They bully people. So, don't feel bad about abruptly hanging up on them. That's the type of behavior they actually understand. Sometimes it's the only way to salvage any kind of relationship at all.