True, but I think it's also a matter of culture. In countries with much diversity, religion does involve quite a bit of self-disclosure, it is understood as an intimately personal choice and maybe it is easier to follow professional guidelines not to bring it to clients.

In my country, however, a lot of people are Orthodox Christians. It's so blended with our cultural context that people normally expect you to be an Orthodox Christian without even asking, in my experience. I have met therapists who actually bring this to the table as a help in therapy - some in an open way just asking if religion might help, others rather bluntly and stating things about God as facts. I think in our culture it seems more accepted for someone (even a therapist) to bring up this subject than it is elsewhere. At least, my experience with religious therapists has been much better and non-discriminatory (I am an atheist myself) than my experience with other categories, such as teachers I had back in high school.