Because when some logical information enters their brain they become terrified that they will be sent to hell if they even think about anything logical . Hell was made up to keep religious sheep from straying from the flock.

They are afraid that their whole life has been built on a lie...a false premise, and that thought is too unthinkable for them to contemplate. The anger and denial are perfectly understandable in that context, because to believe that what we say is true would bring the whole edifice of their life, as they know it, to crumble and crack around them.

I posted this question once on FB and the response was comical. It made me realize just how emotionally childish most believers can be. (not that childish is necessarily exclusive to believers but it sure is a seemingly dominant trait!).

Because their religion is actually FEAR based despite the claims of love.

Mom became a Sunday school teacher when she was eleven . She was twenty eight when I was born , and continued to teach Sunday School , until after I was married . I often find , especially when they come knocking at the door , that they know less about their religion , than I do . After pointing out their mistakes , I tell them to go home and read their Bible . They should at least learn what they're going to preach , before they go out to try to talk others into believing , when they don't know what they're talking about .

When you're part of a herd, anything different rouses your insecurity.

The same reason gays threaten religion I suppose. They are afraid it's contagious.

They’ve had so much religious indoctrination they reject anything that doesn’t agree with what they believe. Hell common sense should tell you religion just doesn’t make any sense.

Because we represent rational thinking

I think it's because, deep down, they know it's all bullshit, and that scares
them to death. To have to acknowledge that your faith is based on lies, and
that your god has never existed in reality, is petrifying.
Atheists threaten them because atheists are living proof that no one "needs"
religion to live their lives happily and productively.

Anathema, in common usage, is something or someone that is detested or shunned. In its other main usage, it is a formal excommunication. The latter meaning, its ecclesiastical sense, is based on New Testament usage. (Wikipedia)

In both cases, atheists challenge the need for belief and religion.
Our existence proves their beliefs are unnecessary to live.

I think pretty much all kinds of people can be terrified when their foundational worldview is challenged.

When your beliefs are malarkey but you hold onto them anyway, you fear even a crack in them might open you up to question everything your faith is based on. And many people have been indoctrinated into believing without faith, they have nothing.

As a Christian of 30 years, I feel that I can give insight on this. Let me start by saying that it’s taken a lot of work on my part to move past this thinking and it’s not how I think now.

It’s pretty simple. Christians are taught that Christianity is 100% true and that you must be in the spirit to understand. Christians look at atheists as people who have been deceived by the devil. They don't know god and are just miserable people. You see it as offering proof that god doesn’t exist and they see you as someone who is lost and going to hell.

Facts really don’t matter to the Christian. You can give the best facts in the world and they will pull out the verse about how god uses the foolish things in the world to confound the wise and that you think you are very smart but hell is littered with smart people.Christians are really not worried like you think that it may expose their religion as fake. They have already hypnotized themselves into believing and they feel that if they even so much as acknowledge your facts then they have committed blasphemy and going to hell. It’s a self defense mechanism to just completely ignore anything that goes against Christianity as from the devil.

You really have to understand the role that He’ll plays in the church. Everyone is so damn afraid of doing or saying something wrong that they are going to hell that even considering for a moment that god is not real is really bad. If you have that thought then you must get it out of your head immediately and if someone is trying to convince you that god isn’t real well then that person just wants you to go to hell with them and must be ignored and chastised. The thought of burning in hell makes people disengage from their rational mind

Christians like to be the underdog. Every christian I have ask to provide objective evidence that god is real, has failed to do so.

Because having your whole belief system challenged, shot down, and destroyed is just too much for some to handle. Especially if they’re the type that has all the answers, knows everything and thinks Themselves secure

Fear in my opinion. Religion attempted to explain the unexplained and the two biggest unexplained items of undeveloped man were birth and death. Where do I come from? Where do I go to? The second, where do I go, that is choked full of fear and to calm those fears the story of be benevolent god with a happy afterlife is created and retold countless times. Some can not handle that death is the end. They need to feel there is something more and that makes them feel safe and happy. Atheism and what Atheists espouse takes away the safety blanket, reality that death is the end sets in and fear returns.

Nobody likes to be told they have been foolish their whole lives. Imagine, how those of us who were once religious would have felt if someone told us we were stupid for believing such garbage. It doesn't stop at religion either and we often get this emotion on this site.

I think it's because your very existence threatens their beliefs. How could anybody not believe in their god? And when they see or talk to you, they have to deal with these feelings that they are not accustomed to (questioning their beliefs). I can imagine it being quite uncomfortable for them. These ideas they've had all their life and have been so certain of are being questioned. How dare you or anyone else question the creator of the universe. The offense.

But anyway, that's the conclusion I have come to about it. Really though, I live in the south and most of my friends are christians and they are not like this. However, I grew up around and still know a lot of people that are just like this.

your question is right on....why do they feel threatened and often act desperately and determined to convince us/me that their belief is right and I should change/come over?

"Why do Christians get threatened and terrified by Atheists?" The short answer is I don't know, and there may be several reasons, not just one. But I have a theory that might explain some of it. It is similar to a theory about why some straight married people say that same-sex marriage weakens marriage and is an attack on their marriage. The theory is that they are not sure they really are straight, that they fear that they themselves may be gay, or leaning that way. Similarly, religious people may fear that the atheists are right. Your last sentence sums it up: If you actually believe everything in the bible is the truth, how can you feel threatened? Because you don't really believe that everything in the bible is true. If you really study the issue you should know that the bible is terribly flawed. Actual biblical scholars know that the bible as it exists today isn't all true, and cannot be. For example, read Bart Ehrman's "Misquoting Jesus" where he explains (and actuall biblical scholars agree with this) that the bible you read today bears little resemblance to the actual original texts as originally written.

When one constructs their entire identity as a human being around a narrative, they will naturally revert to defense mode when this narrative is shown to be false. There is too much at stake for them to admit defeat.

To be fair, you are relating to a very small number of Christians, probably Amero-centric. I have never met a Christian who considers that the Earth is 6,000 years and the dinosaur relics are the devil playing a trick on humanity.

These are, I suspect, the gullible people who need something to believe in and have been sucked into the evangelical black hole of suspension of reality.

It is a bit silly when you consider that most of the oppression that Christians have suffered has been from other Abrahamists / Christians. Even the often touted "War On Christmas" is almost exclussively from other Christians. Most of us Atheists enjoy the Winter Holidays just fine, reguardless of the mythology.

Because you are ruining their buzz.
Religion is nature's anti-depressant: Narcissistic delusions of grandeur and significance, embraced to give yourself a feel-good-chemical bump when reality is letting you down.
Of course, most people are "embracing" it too excess and have become addicted. Telling them it's a lie will not break them free but it will hurt them - the same way withdrawal hurts addicts - until they can find a safe-church-space with like-minded addicts to shoot up together.