I'd wager there are a great many reasons. Just speculating I'd say:
People, all people, respond to many things personally and emotionally when it makes no rational sense. I think it's just in the wiring to a certain degree.
ah! I fellow redditor! i usually post both places hahaha and usually get two different sets of answers, both very good :)
I assumed people I have experienced like that believed it was their duty. Sort of like they were getting brownie points with god or something to speak out. I think Kris nailed it.
Because believing in a god is the basis for their entire way of life. They really think morality comes from religion and without religion we are without morals. By not believing you are, in a way, saying that they are wrong, that god is a fabrication, and that the bible is a work of fiction and full of lies.
thats how my husband, Chris, step dad gets.
His head is so far up the bible's ass he has to rub it in Chris face when ever he sees him lol
They identify as their belief instead of as an individual, so anything that can be perceived as an attack on their beliefs is immediately translated to a personal attack. Given that existence or non-existence are the only two options, they don’t like the insinuation that their belief is wrong, but have no problem telling you that yours is.
Perhaps they feel as though we're mocking their beliefs, rather than simply choosing not to share them?
Perish the thought.
Those that take it personally have an emotional attachment to their religion or cannot bare the consquences if they are wrong.
People are idiots.