The complexity of partners of different religions making a life-long relationship successful, doesn't really get tested until there are children involved. While one partner may be able to overlook the "details" of the differences between their beliefs and that of their partner's, how does that work with children? How to educate children in one or both religions without turning them into athiests and/or hypocrites?
Before I got married, I asked my husband if he believed in God. He did, and that was enough for me. He was of a Christian faith but not practicing, and I am Muslim. I think I would have had some difficulty being married to someone who was an athiest or agnostic. Although I have a very good friend who is agnostic, I think I am too worried about my own salvation to be too concerned with hers...Hmm that doesn't sound very good. But anyway, my husband and I decided, if we had children, then the partner practicing their faith would be the one responsible for religious education. It seemed a good idea at the time considering it was all hypothetical. My husband felt comfortable about not eating food that I would not want to cook, and I was not prevented in any way from practicing Islam. That all worked well until my husband rediscovered his faith.
Now both of us are active following our own religion. My husband goes to church, and Ramadan is starting so I will be fasting. The challenge is to educate our children on the similarities of both Christian faiths and Islam; being respectful of what others believe; sharing our knowledge about our faiths and traditions we grew up with; equipping them to be able to make their own decisions about faith when they are older; and above all, trying to raise children we can be proud of, regardless of their own religious (or non-religious) path.