I don't know much about his mother's faith, but that sounds very plausible.

I'll copy what I said to chris_baby: In one interview -- I think it was on radio, because I remember the audio but no video with it -- Stephen said this about speaking at his daughter's confirmation: "It gave me the opportunity to speak very simply and plainly about my faith, which is something I don't get to do very often without people saying, 'Yeah, but do you really believe all that stuff?'"

So I can see why he's reluctant to talk about it in public venues. His fans and his audience skew skeptical, and it can't be fun to talk about it if he has to preface it with "yes, I'm actually Catholic; yes, I'm aware of the illogicities and contradictions; yes, I'm aware of the atrocities committed in the name of religion; no, I don't condone them; but yes, I do believe these things..."