Why do the religious get so pissy if you say their religion isn't true? If they are really so sure it is, why do they care?

This never made sense to me. If they are really as sure as they claim they are, they shouldn't feel threatened at all by disbelieving it. Think about it this way, you know for sure the sun exists, right? Imagine if someone told you "the sun isn't real. It's just a myth", would you think "How dare you say that! That makes me so mad!"? Or would you just think "yeah, he's insane". Obviously, it would be the latter.