Oh, I can VERY easily answer this.
It's because religion can cause harm.
Some people are abused because of it, whether it be through priests (supposedly "holy men of God") or their own families.
Women were seen as inferior because of the Bible, slavery was perpetuated and given Bible verses in support of it. And there are many.
People, even a few on this forum, are deathly terrified of hell. It sickens me to see people here that afraid. And hell is such a barbaric concept that it seems only evil people would think about perpetuating such a thing.
Some things in the Bible are completely awful, like God commanding Genocide, slaughter but keeping the virgins for themselves, beating a slave to death being okay as long as they do t die right away, and so many many other things.
The doctrine of original sin can cause some to have incredibly low self-esteem.
It can cause people to make decisions that turn out awfully because they think it's what God is telling them.
Questioning can bring ridicule or hostility from strong believers and cause the doubters to feel shame, guilt-tripped, depressed, and threatened. As if some are trying to scare them into believing.
THAT is why nonbelievers and questioners care. Because it can be heavily damaging, threatening, awful, abusive, and borderline evil. Are there good points to religion? Sure, with the right people, but points like the above are terrible things that people need to know about and shouldn't be scared from questioning or feel shame, guilt, and afraid of their own thoughts as a result.
So, what gives them the right to hurt and attack something I hold so dear? I'm sorry they were hurt by religion, but they want to repay hurt with hurt? That's a morally acceptable thing to do?
I think a lot of people get the idea of Christianity wrong. I see atheists going around saying untrue things, like most wars have been fought over religion. That's not true at all. Most wars have been fought over land and resources. The biggest war in world history, taking the lives of over 60 million people, was due to one man's diluted fantasy of a genetically superior race, so he attempted to rid the world of inferior species. That sounds more like a mad scientist trying to play evolution than a holy war the atheists try to make it out to be.
It's funny how politics shapes things today. A liberal or atheist can look at Muslims and go, "You know what, most are good. Most of them want nothing to do with barbarism the extremists display, but for the most part, Islam is a peaceful religion." But when it comes to Christians here in the States, we're looked at as the mighty oppressors. There are plenty of liberals who are Christian, but we're seen a greedy, mean people who want to take away rights and support slavery, etc. But again, it's not true.
Christians are the most generous people on the face of the earth. We give away more money to help out than any other faith, country, or creed. Christian nations give more than secular nations. The majority of the charities in the world are Christian charities. Here in America, we give much more to our poor (even though it's often not enough). We take in the sick and the hungry, even the illegals. We spend $113 BILLION per year taking care of illegal immigrants, feeding them, clothing them, allowing their kids to go to school, housing them, giving free health insurance, a free car, etc.
I'm sorry, but Christians aren't these bad, evil people. You feel oppressed because we think homosexuality is a sin? Yeah, there are a few bad apples out there, but I've never gone up to a gay person and shouted "SINNER!" in their face. I've never attacked someone for their sin or judged them. I believe what I believe and if you're going to come at me like you're goal is to rid my faith from me, of course I'm going to be hostile. Religious beliefs are sacred to a lot of people, me included.
It's a bit hypocritical to ask why Christians are being hostile when it's the atheists who acted hostile first. Now, if someone kindly PMd me or asked a question about God, I have no issue answering the question. I'm respectful and civil if they're civil and respectful back. If someone wants to debate me on the forum here, I'm all for it. Let's open the door for civil conversation, but if their only desire is to get me to disbelief and they're hostile and calling me an idiot because I "believe in a sky fairy", I'm not going to deal with it.