As a fellow 16 year old, I don’t really think it’s necessary to think about marriage at this point. It’s probably best if you don’t think of it as that serious yet, (not that I follow my own advice, of course, but I try

). If you like her, and it’s not causing you any additional problems, I don’t think you necessarily have to break up with her. It’ll ultimately come down to how much of a problem her lack of faith is for you. I don’t think I could seriously date an atheist for long, but that’s me.
I will have to think about this at some point, too, because 98% of the young men I know are Lutheran. I don’t know if dating outside of my faith would be a good idea, but I also think that I need to start getting used to interacting with guys.