He may not think his opinions are offensive or unwelcome...
It's all a matter of perspective.
I'll grant that. However, reasonableness would seem to dictate that one would know that the statement, "Only an idiot believes in X" would be offensive to those who believe in X. He may not be reasonable, though.
How many religious expression claim that you will be going to hell for one reason or another. In their perspective, they are saving my soul. In my perspective they are being delusional. We are both entitled to our opinions. I keep the peace by refraining from commenting. It's not easy to do, but I feel it is the most polite thing to ignore inflammatory opportunities like that. The more unreasonable the opinion the less likely the chance for common ground--so why waste your time?
Good point. The words sinner, infidel, heathen and so on didn't invent themselves, yet I don't suppose that believers want to be prevented from using them now. I find them no less obnoxious than referring to believers as "gullible" or "delusional." It's a two-way street but generally extra deference seems to be expected toward the faithful.
Will bow out now before this thread is locked for addressing religion; I'm really just trying to stick to the way various camps use language and when it crosses from annoying to rude.