Why does she want you to marry her?
Why did she move out after living with you?
It may be because of family pressure and the shunning is working. If you get married then her family may re-engage with her and that could be a strong motivation.
You need a series of heart to heart conversations and she needs to be willing to honestly investigate her religion (cult) of birth (i'm guessing she was a born in).
If she is not or will not be willing to do that then walk away.
Edit to add: if she read this whole thread, what would be the result? It's possible the indoctrination is still so strong that she wont even read anything on an "apostate" site like this one.