The crux of the matter is that religion is a very personal matter. Many people have strong feelings one way or another about their religion.
Religion is also a personal pathway in a person's life, and when one is made to feel outside of their personal "comfort zone" or that their pathway is being challenged, then they may become uncomfortable about the person(s) who made them feel that way.
If I were an F/O I would not like to feel "uncomfortable" around the Captain. As a passenger I want to do as little "thinking" about the Captain on the flight as possible.
It may be nice for the other (in this case) christians on the flight, but what about the feelings of the others?
There are no grounds to compare Saudi Airlines to this situation, in many Islamic countires the way of life revolves around their religion and their religion can be observed in most daily events. This is not the case in most western cultures. An event like this is expected on Saudi Airlines, not quites so with AA ??
Invictus