(Disclaimer: This post is sympathetic to a certain subset of theists. I am not myself a theist, nor have I ever been one. I do not intend to justify all varieties of theism, nor do I intend to justify much in the way of common theistic behavior.)
I'm not adopted. You all believe me, right? How do you think I came by this information, that you're confident in my statement? The obvious and correct answer is that my parents told me so1. Why do I believe them? Well, they would be in a position to know the answer, and they have been generally honest and sincere in their statements to me. A false belief on the subject could be hazardous to me, if I report inaccurate family history to physicians, and I believe that my parents have my safety in mind. I know of the existence of adopted people; the possibility isn't completely absent from my mind - but I believe quite confidently that I am not among those people, because my parents say otherwise.
Now let's consider another example. I have a friend who plans to name her first daughter Wednesday. Wednesday will also not be adopted, but that isn't the part of the example that is important: Wednesday will grow up in Provo, Utah, in a Mormon family in a Mormon community with Mormon friends, classmates, and neighbors, attending an LDS church every week and reading scripture and participating in church activities. It is overwhelmingly likely that she will believe the doctrines of the LDS church, because not only her parents, but virtually everyone she knows will reinforce these beliefs in her. Given the particular nuances of Mormonism as opposed to other forms of Christianity, Wednesday will also be regularly informed that several of these people are in a position to have special knowledge on the subject via direct prayer-derived evidence2 - in much the same way that her parents will have special knowledge of her non-adopted status via direct experience when she wasn't in a state suitable to notice or remember the events. Also, a false belief on the subject could have all kinds of bad consequences - if the Muslims are right, for instance, no doubt Hell awaits Wednesday and her family - so if she also correctly assumes that her parents have her best interests at heart, she'll assume they would do their best to give her accurate information.
Atheism tends to be treated as an open-and-shut case here and in other intellectually sophisticated venues, but is that fair? What about Wednesday? What would have to happen to her to get her to give up those beliefs? Well, for starters, she'd have to dramatically change her opinion of her family. Her parents care enough about honesty that they are already planning not to deceive her about Santa Claus - should she believe that they're liars? They're both college-educated, clever people, who read a lot and think carefully about (some) things - should she believe that they're fools? They've traveled around the world and have friends like me who are, vocally, non-Mormons and even non-Christians - should she believe that her parents have not been exposed to other ideas?
Would giving up her religion help Wednesday win? I don't think her family would outright reject her for it, but it would definitely strain those valued relationships, and some of the aforementioned friends, classmates, and neighbors would certainly react badly. It doesn't seem that it would make her any richer, happier, more successful - especially if she carries on living in Utah3. (I reject out of hand the idea that she should deconvert in the closet and systematically lie to everyone she knows.) It would make her right. And that would be all it would do - if she were lucky.
Is it really essential that, as a community, we exclude or dismiss or reflexively criticize theists who are good at partitioning, who like and are good at rational reasoning in every other sphere - and who just have higher priorities than being right? I have priorities that I'd probably put ahead of being right, too; I'm just not in a position where I really have to choose between "keeping my friends and being right", "feeling at home and being right", "eating this week and being right". That's my luck, not my cleverness, at work.
When Wednesday has been born and has learned to read, it would be nice if there were a place for her here.
1I have other evidence - I have inherited some physical characteristics from my parents and have seen my birth certificate - but the point is that this is something I would take their word for even if I didn't take after them very strongly and had never seen the documentation.
2Mormons believe in direct revelation, and they also believe that priesthood authorities are entitled to receive revelations for those over whom they have said authority (e.g. fathers for their children, husbands for their wives, etc.).
3I have lived in Salt Lake City, and during this time was, as always, openly an atheist. Everyone was tolerant of me, but I do not think it improved my situation in any way.
Should you be Wednesday's "atheist auntie?" I would say yes.
You're asking, "What does she gain from being an atheist?" Well, there are several possibilities -- someone mentioned a happier sexuality -- but, in my opinion, what really matters is the end of the divided will. Sooner or later, most people find some tenet of their religion that they disagree with, or think is silly, or even horrible, but they're convinced that God wills it. How do you disagree with God? Well, in my case, for a long time, my basic moral premise was "I suck." That's no way to live.
Religion can work very well for people who can compartmentalize, or not take it too seriously. Most religious people treat it as a pleasant tradition and an impetus to do right, and that's pretty much okay in my book. Not everyone is a big fan of consistency the way I am -- I have a rather black-and-white personality. But if you are a stickler for consistency then religion will break you and terrify you, and actually prevent you from living well.
You do her no harm if you're her "atheist auntie." If she stays a Mormon she'll just be a cosmopolitan one who can say "some of my best friends are atheist," which isn't so bad in itself. If she doesn't -- well, she may have to endure secrecy (while her parents are still supporting her) but she may be better off for it in the long run.
Should we be nicer to theists? I actually have a pet peeve against smarmy atheists. That can be just another bias. There can be a snotty, ignorant tone -- less often here, but familiar from people I know in real life who were raised non-religious. I don't like it, it serves no good purpose, and maybe we should consciously avoid it. Stop and think for a minute and there are historical and present-day heroes who believe in God.
In my experience, people in non-fundamentalist religious traditions tend to change their God to match their new opinions.
(That's no way to live either, but at least it can be less mentally burdensome than blaming oneself.)
Edit: I'm saying that sincerely, and I hope it doesn't come off as smarmy/ignorant atheism. It's an accurate description of most of the liberal religious people I know.