You aren't engaged or married or anything, you can't just know that she won't see the light and remain crazy Mormon forever.
Tell her that you want your relationship with her, and her relationship with your children together to be separate from her religion with her god. Sometimes I complain to my kids and to my husband. I spent a lot of time on my knees and made several trips to the temple before I felt l could trust that what I knew I wanted to be promptings actually were.
I remember when my ex and I had invited friends to our house for a 4th of July dinner and had spent a lot of money and time getting ready.