I just can't see the relationship between religion and photography. Why would someone prefer a religious photographer, if the subject probably won't come up in the first place?
It's not about religion per se, it's about getting on with the client. If you're covering weddings, baptisms, confirmations, bar mitzvah, etc you're covering a religious ceremony. A person who shares the same religious beliefs will connect better with you and you will get a long better. Let's take a different example, as the religion bit is a red herring.
Imagine that you're a band and you're after some publicity shots. You've got two photographers, both equally competent, have decent portfolios and are passionate about photography. One of them is also a musician, plays in a band, and is into the kind of music you're making. The other photographer, well he listens to music but doesn't play an instrument. Which photographer will you get along better with, and as a consequence be more likely to hire?
You get along best with people who are like minded.