You're a grown man - you need to set boundaries with your mum. You should have done this years ago but I understand the persistent thought-control mother type and it's never too late to get your space back. She is indulging in a pattern of behaviour encouraged by the JW's - "witness more and you will save him". That pattern of behaviour is destructive as you're finding it annoying, invasive and not answering all her calls.
Step one, you need to stop this bad pattern of her constantly preaching to you and invading your privacy and set boundaries. You can say you love her very much and appreciate she thinks she is trying to help you but this is unacceptable behaviour and you need some space. You do not want to discuss religion with her as there is nothing she can say that you have not already heard and that she needs to respect you are a grown man now.
Tell her that she is free to call a few times a week, in fact why don't you offer to call her and and instead talk about other topics that are neutral. It is important you try and make these calls positive and maybe plan some things to talk about. If she starts to talk religion and oversteps the mark then remind her you'd both agreed not to talk religion and change the subject or cut the conversation short very quickly. It may seem cruel or disrespectful but it will change her behaviour.
At the moment your mum is thinking I need to call him or he will die at Armaggeddon. Then when she hangs up the phone she is frustrated she hasn't saved you yet. It is a very negative cycle, it's not healthy and a terrible relationship to have with someone. So being firm is going to help you and it will help her too. She will still feel frustrated but you will start to have a positive relationship.
The reason why it is important you let her know where she stands now and change the dynamic of your contact is that if you get married it will become a big problem with your future wife when your mother keeps invading your privacy.
When you start to do all this she will know you mean business and start to respect the new boundaries - albeit with some kicking and screaming along the way. It will then mean she will also respect the boundaries of not talking religion with your future wife when you make it clear your wife also does not want to talk about it.
It means you future wife can build a reasonable relationship with your mother without futile debates on religion that will end in tears.
It sounds like you have found a wonderful woman, if you man-up now life will get easier. Clearly there will be future issues about the wedding and if there are kids but that is for another day. Congratulations, I with you every success.
mmxiv