Two seperate belief systems, one of you may be non religious? Religious differences can be a right deal breaker, you may have ‘came out’ later in the relationship after that lovely (yet crucial) honeymoon period! It could have been the case in a previous relationship that didn’t work for this very reason.
How do you make it work? Maybe you live in a country in which a mixed of faiths would not be possible and therefore had to find someone who shared your beliefs, or risk rejection?
Atheists too. I am sure there are many atheists that would not consider going out with someone of religion, and vice versa.