The advice from posters such as "Witness My Fury" and "DJS" might seem blunt and harsh, but it is the best advice.
It is hard for an outsider to understand the grip the cult. You may think that if you use the right logical explanation, the right information, the right biblical interpretation, etc you can get her to see the cult for what it is. You will NEVER succeed with intellectual reasoning.
You would be better off doing some research on how cults control minds at an emotional level, and learning what you are up against.
If you end up marrying this girl, this will be your life. You will eventually find that you will not be able to attend birthdays or xmas functions any more. She will be gone 2 times a week to attend meetings. You will be under constant pressure to attend them yourself. Social ties with others become strained, because she can't attend so many things. Then you find that the only people who you can mix with together are the cult drones. You will start to notice that the cult drones are friendly in the way insurance salesmen are friendly; always looking to offer a product you don't want. Those cult drones will often be vacuous airheads, and you will notice that a lot of them depend on some form of social security to get by. And then if you have kids, it gets a whole lot worse.
Therefore, save yourself a life of misery, and walk away. It may be painful now, but you will really regret it later if you don't.