You asked if calling someone's belief "crap" was disrespectful. I maintain that it is not. Your statement, however, calls people "stupid" (which is a direct insult) and uses a derogatory term for people speaking their mind ("spew"). I'll admit that the second one is more a matter of distaste rather than disrespect, but the first is clearly disrespectful to a person.
We all know that calling someone stupid is an insult. If you go back and read my post above, you'll find that I didn't insult anyone.
Let me see if I can use a somewhat hyperbolic example (let's call it enraged for contrast) to make my point.
Meet John. John is a Pentecostal. John believes that God speaks to him personally, and more importantly that God sometimes speaks through people in a holy language that only those "touched by the spirit" can understand. John believes that you, too, can be "touched by the spirit" if you believe hard enough.
Meet Jane. Jane is a Southern Baptist. Jane holds no truck with those "Holy Roller Types." Jane believes that God doesn't speak through people in babble-tongue, and that such demonstrations might even be the work of the Devil himself.
Jane meets John in the lunch room at work, and as they both discuss what they did over the weekend, the topic of church comes up and they begin a discussion about religion. John tells Jane that he's been "working on" his faith for a long time, and that he is just as happy as he can be because finally, last weekend, he was "touched by the spirit." John asks Jane if she's ever been "touched by the spirit." Jane tells John, honestly, that she believes no one is really "touched by the spirit," and that if there is anything at all "real" to what John experienced that it's probably the work of the Devil.
Choose your own adventure: How should John respond?
A. "Oh that's crap!"
B. "You're an idiot if you believe that!"
This little morality play has several points.
1. A is what I posted above. B is what you posted here. Do you see a difference between them, because I certainly do. Telling someone they are wrong, even if you do so strongly or even in-artfully, is not the same as insulting someone directly.
2. More importantly, if you think both of these choices are disrespectful, then wasn't Jane disrespectful? She did, after all, imply that John had been touched by the Devil.
3. If this conversation between two Christians who disagreed contained "disrespect," then we are completely fucked in the realm of religious discussion.
4. Was the workplace really the best place to start a conversation on a subject known to be divisive?
"Disrespect" is a smokescreen. It's the secret option "C" above, where John could have said "that's disrespectful!" It shuts down the conversation completely while insulating the claimant from any criticism (right or wrong) of their belief by allowing them to claim victim status.
It's the "PLEASE STOP" from Hyperbole and a Half. The kicker paragraph in that story, of course, is the last one:
As we grew up, we learned to solve our problems through "talking" and "compromise," but I think secretly we all still yearned for the days where we only had to yell "PLEASE STOP" and anything we wanted was ours.
Also, when you and your fellow hosts ban the one with Stalin-Tourette's, we can talk about the "Don't make DU suck" rule.
The whole point of the No True Scotsman fallacy is that being a Scot is a matter of birth. The name comes from an allegory used to illustrate the fallacious action of redefining a term in an ad hoc fashion to suit one's own rhetorical purposes.
The reason you see this allegory referenced so often in the religion forum is that Christians simply cannot resist engaging in this fallacy. They redefine what it means to be Christian in an ad hoc fashion every day in order to suit their own rhetorical needs. They have to, because every day they are confronted with some self-righteous asshole with one hand on the Bible, the other somewhere it shouldn't be, and hate on his lips.
Of course you redefine the label to ensure that he's outside the boundary, and you say "no true Christian would ever diddle kids/steal from the collection plate/swindle the elderly/ignore the poor/be a conservative." The problem there is two-fold: You've fallaciously redefined the label, and you have absolutely no universally accepted defnition for the word. There are people here on DU who call themselves Christian and don't even believe that Jesus was actually a man you could call Christ (Annointed One), and who are you to tell them that their self-identification is wrong?
There's no such thing as a "Paulist", kwassa. You made it up, and you did it by engaging in a fallacy, and redefining Christianity so Paul's words fell outside your ever-moving boundary.