A question I see regularly. Here’s my take, as a Christian (who is routinely embarrassed by my fellow Christians’ misdeeds).
A lot of us are raised to believe, explicitly and fervently, that doubt is an existential THREAT. And even if we aren’t explicitly taught to hate and fear outsiders, it comes across implicitly in a number of our practices, beliefs, and philosophies.
Which makes sense, when you think about it. It’s human nature to form groups, and for millions of years, the only way we could survive was to stick with a group and view outsiders with wariness and treat them with hostility as needed.
To Christians, this is what atheists and practitioners of other faiths and even different denominations are: outsiders who present a potential threat.
Practically, this threat is rarely physical, although plenty of churches (particularly insular/conservative ones) preach that atheists, etc, intend to kill us all or take over the country or replace all “Christians” (they usually mean white people).
The threat is more psychological.
Many Christians believe the only way to be a moral, good person is to be a Christian.
Which is, of course, nonsense that I’ve spoken about before. But this implies that non-Christians are inherently immoral and potentially dangerous, and someone who’s been indoctrinated from an early age to believe that will often react poorly when presented with such a person. They will try desperately to convert that person or dismiss them, because of the deeper threat they pose.
The symbolic threat.
For you see, if a non-Christian is indeed moral, compassionate, sensible, even kind, that shakes them to their core. That’s supposed to be impossible, and the fact that it IS possible means their insular church LIED to them. And what else did they lie about?
Encountering a kind, moral, upstanding atheist can be the thread pull that unravels the entirety of a Christian’s faith. That almost happened to me, in fact, but fortunately I hail from a fairly liberal United Methodist church that prepared me to accept and incorporate this new perspective.
Same thing happens when kids go to college, actually. They encounter new people outside their community, and the more insular their community was, the harder that culture shock is. College students come home at winter break with new ideas not because they’re being indoctrinated—but because they’re finally not living in an environment steeped in indoctrination.
And here’s the thing: when Christians from an insular, conservative faith community realize there are other ways to live, that might force them to confront the years they’ve spent sacrificing and costly signaling. If they meet an awesome gay person and realize homophobia is bogus (and blasphemous), what of the years they spent hating gay people and pretending the Bible told them to? All that hate and condemnation was not only a waste, but actively against Jesus’s will.
And gay people are only a slight challenge compared to atheists, who call the whole thing into question.
People are naturally inclined to double-down to protect themselves from cognitive dissonance. And keep doubling down, no matter how ridiculous their position becomes, or how logically twisted or distorted their arguments. Their very sense of self is on the line.
So… if you’re an atheist, be patient with Christians, if you can. You’re dealing with a fear response—where they feel the desperate need to reassert their faith in the face of a source of doubt. Some of us are flexible and can evolve. Some of us… it takes a little more work.

Further reading: some faith traditions actually use this process as a means of reinforcing the faith of their adherents, by putting them in mostly controlled confrontations with doubt for which they are as prepared as the church can make them. It’s a fascinating process.