Anything can be a religion to you if you make it that. For some it is the theory of evolution, for others just believing in the lack of anything greater than humanity, or for others even money. You can idolize anything and thereby make it your god, your religion.
As for your question, I would never recommend people with two different religions tying to have a relationship together. Even when you are respectful of each others' beliefs, you will always come back to the point that they are not the same. If religion is the most important thing in your life, then you will have to favor it over your relationship, and this will not help the relationship. If you decide just not to talk about what you believe, then you'll probably be giving the back seat to religion, which may be in conflict with what you believe.
I myself am a Christian, and I had a relationship with an Atheist. We were both very respectful of each others' differing views, but in the end we realized that religion was a pretty darn HUGE thing not to have in common. It was the main reason why we broke up, regardless of however insanely polite we had been toward one another. Now, it's impossible for me not to see the situation with some hindsight bias, but I would avoid the relationship and prevent the pain if I could make the choices that led to them over again.
That was my experience. You are still free to decide as you wish, of course, but it's always nicer to learn from others' mistakes than from your own. ~_^
“Hey, lemme borrow your bike. C’mon, I’ll give you some chips.â€