My experience with this is interesting. While I've never really been in a romantic relationship of any kind (let alone with someone who has different religious beliefs), my parents set an awfully good example of what we're talking about here. My mother is a rather devout Christian. Not like the kind that thinks that going to church and giving tithe and reading the Bible every minute of every day kind of Christian, though she certainly believes that those things help. She's the kind that knows the gist of what the Bible says, skips the details, and thinks that the most important part of her religion is having a personal relationship with Jesus. Now, just in my experience, the kind of Christian that my mother is far more sincere than the kind that thinks that going to church is the most important aspect of their religion, so I believe that my mother is what I classify as a "true Christian."

My father on the other hand, is an agnostic with some pretty atheistic tendencies. He's still on the fence, but if someone were to shove him, he would definitely fall over onto the atheist side. That being said, watching their relationship was rather interesting growing up. Though I didn't understand it when I was nine, being nineteen now it's kind of a different story, though I still don't understand all aspects of their relationship. And hey, why would I? It's their relationship, not mine. What I do understand is this: my parents love each other. They almost got divorced about nine years ago, but they got over their differences and have been happily married since.

Now, when I say happily married, I of course don't mean that everything is dilly-dally nice and joyful. It means that they are happy to be together, despite the fact that they often disagree on things. Now, admittedly, the majority of their arguments stem from their differing world views, which are definitely influenced by their respective religions, or lack thereof. So, it could be said that if my father was Christian, or my mother was agnostic/atheist, then they wouldn't disagree on nearly as much (although since atheism/agnosticism doesn't really have set rules, there would probably still be a fair amount of arguments) as they do now. But would they have really gained anything from each other? Probably not.

Both them, and their children including me, have been able to learn from their relationship. My mother was a pretty liberal person before she met my father, but she probably never would have really understood where pro-choice and homosexual marriage advocates were coming from if not for my father. On the other side of the coin, my father probably wouldn't have went through life thinking that all religious people are irrational nut-jobs, but my mother showed him the other side of Christianity, and that not all of them are insane people who just walk around saying, "God, god, god, god, god!"

Also, their children have benefited from it, myself included. It has given all of us a very rounded world view. Now, while I eventually ended up as an atheist, I went through several stages of religion before I wound up there. I started out as a Christian, because my mother insisted on taking all of her children to church. Then, around sixteen, when I really started to start making important decisions on my own, I started to understand my fathers point of view as well, and became an agnostic. Then, as I realized that I shouldn't follow either of my parents, but instead go find a religion that suited me the best (which was most definitely spurred by the mixed religions in my house), I was almost Buddhist, an even closer to almost Mormon (no offense to any Mormons on this board, but I really feel I dodged a bullet there), and then finally settled on atheist, which I have stuck with for just over a year and a half now.

My youngest brother is also an atheist. My second youngest brother doesn't really know what he is, and my sister is a Christian in very much the same sense that my mother is. A true Christian, in my book. And then when you think about all of the people that any of me or my three siblings have ever interacted with have at least shared in some aspect of our world view, whether they agreed with it or not. So in the end, my mother and father deciding to marry each other despite their religious differences has affected them and pretty much everyone around them for the better.

So, in closing, while I think that if religious differences really do just make the relationship too hard, that eventually people just need to know when to quit. But if they can put up with it, I agree with Hombre that they really should. Just look at how many benefits have been reaped from my mother and father's marriage. And that's only one example! So in the end, try, but if it really isn't going to work, you just need to know when to call it quits.