I'm not going to bitch about if you're a "true" Christian or not since that's not what you asked unlike other people but like who cares what religion they are? If I find the girl of my dreams /cringe/ then I'd date her if she was a different religion to me
You can't be a very devout Christian if you're having premarital sex so I don't really see why you are questioning disobeying another Christian value and dating outside your religion since you don't REALLY respect your religion anyway.
I mean, it doesn't seem that either of you are religious. Both of you hooked up and are hooking up again, she's not following Islam neither are you following Christianity. Religion should be kept outside of the equation and you guys are compatible when it comes to religion.