We've all heard that "Anger is only fear directed inwards."
What this means is that people get angry when they fear something internally
My guess is that people are offended (angry) when they hear something that they fear is true. I'm not offended when someone calls me gay or stupid, because for one, nothing is wrong with being gay, but more importantly I know for a fact that those things aren't true.
This goes the same for religion. I'm not offended when someone says there is a god, but the religious are deeply offended when someone says there isn't a god, or even that there is one but their god is not the correct one.
My guess is that they're offended because of an internal fear.
Let me know your opinions in the comment section. Please be civil, everyone.