I agree with what you said in the post, above, except with respect to the quoted portion.
Steerpike,
The full sentence reads as follows:
If you disagree with another person's belief, don't bring it up. If your mind is already made up that they are wrong, there's no point in having a discussion. It only puts them on the defensive.
If you initially disagree with a belief, but are open to the possibility that they may be correct, that's a different story. If you are legitimately interested in learning where they are coming from, that's also fine.
But if you KNOW that you're right and that they're wrong, and your objective is to prove how right you are, then having a discussion is pointless.
I'm not sure why someone would feel threatened or unwanted or somehow less able to contribute to the community at a whole because of a thread in which some members disagree with their religious views (or lack thereof). I haven't seen the PMs you are getting, but I've seen them on other boards and they often are some variation on "I don't like or am not interested in that subject, so please don't let people talk about it."
The private messages that I've been receiving are nothing like that.