I have seen quite a few religious IAmAs, and, as a rule, everyone was respectful. Reddit mostly has a problem with people who try to undermine rationality and critical thinking in the name of religion, not with religion itself. If you show me a post where redditors are being disrespectful to a (sane) IAmA-er, I will be very surprised.

In fact, I remember a recent post by a theologian, it had hundreds of comments and everyone (including the poster) loved it, it was very civilized and informative.