I had the whole "marry my first boyfriend" thing going on, but, three ex's later, I've found that's one of those promises to yourself you'll just have to break. All you can really do is give her a chance. You guys seem to get along well enough, at least for now. So long as she doesn't drag you to church all the time, you're cool with the religion thing, right? So long as both of you don't make too much of a fuss about it, it shouldn't be too bad. Actually, one of my friends technically has two religions.XD One of her parents is Jewish and the other is Christian, and they've been rather content in their marriage for about 20-ish years now. It's kinda sweet.<3
Anyway, the religion gap is as much of a gap as you want to make it.
Be careful, though. She seems to be the type to flirt with other guys, even when she's in a relationship; make sure to keep tabs on this. Her ex might've been an ass to you, but, judging from his point of view, he was definitely on to something. She was eyeing other guys(namely you) even though she had a guy already. Banning her from talking to a guy she was flirting with may seem a bit harsh, but it's understandable considering his position at the time. Think up some other ways to keep her eyes on you, especially if you guys start going steady.:)