The "shut the fuck up" mentality comes from people refusing to accept or even listen to points made from the opposition. At a certain point, people get tired of hearing the same arguments over and over again.

"Man wrote the Bible."
"Man made it up."
"Man made up those stories."
"Man made up Heaven."
"Man made religious images."
(That's the same argument.)
"I've never seen God."
(That's just ignorance. You can't argue against anything using ignorance...)

What is it they say about silence being golden?