Hey!
Well, I can appreciate that cultural and religious differences can be hard on relationships.
I think it is important to consider how serious you guys were as a couple.
It sounds silly, but there is naive love that yes, is nice when you're both fairly young, but not going to last forever, lets face it.
Then there is genuine love. If you both genuinely share strong feelings, then I would consider going to considerable lengths to make it work.
There are two families involved really. Hers and yours.
I'm presuming hers are against her having relationships with Christians.
Try talking to your family. Tell them how you feel about her, and what the situation is. They are probably more experience when it comes to these situations. If not, try your close friends- or even her friends.
You need to try and find even the smallest indicator that this could work.
I'm sure her parents have considered that she may fall in love with someone 'not to their choosing', but that doesn't necessarily make it impossible.
It does make it difficult, but if you both want this, what have you got to loose.
You need to talk it over with her. Be as decisive as you can, without being too drastic.
Obviously, you can't go barging in demanding to be with their daughter, 'Praise the Lord!' and all that. Yes, it can be frustrating I imagine, to be forbidden to have a relationship with someone you love.
But, don't let something important go. If you BOTH need each other, talk to one another!
If she refuses to talk to you about your situations, you can't do anything.