Hey there, Parmenides. I am totally cool with you venting at me.

And if it did get out, then we'd risk losing virtually all of our (close-knit, wonderful, highly supportive) families and friends.

I take this especially seriously. Leaving the tribe is hard, especially when it has tangible benefits and costs. I think this is the biggest thing that the rationalist community has yet to fully address insofar as it seeks to compete with other communities in traditional domains, but certainly not for lack of awareness. I think I'll link this video I found of William and Divia Eden's wedding ceremony.

I kind of feel like a creep for doing that, but this is a great example of how rationalists are making their own communities and institutions and rituals. Eliezer makes a bunch of science jokes and implicitly jabs traditional everything, as he is wont to do; the spouses agree that they totally love each other and are in it for as long as they both think they should be and that both of those things are cool; they keep the usual wedding trappings because wedding trappings are fun, and fun is cool; they change their last name to Eden because Eden is a cool last name; and there's a general feeling in the air that being cool about most stuff when it's cool to do so is generally the coolest way to go. Basically they do everything possible to avoid the kind of shitty problem that you're in now. (That is not to say that you could have avoided it through some superior exercise of personal integrity.)

You might also dig these Skepticon panels on how rationalists deal with relationships and death. I highly recommend the one on relationships because there's an atheist on the panel who to my knowledge is a former fundamentalist Christian and is in a relationship with a woman whose entire family are devout Christians.

I say all of this because you can find a new community or have a hand in making a new one. LessWrong is one such community. I have said before that most LessWrongians are 'super smart and super ethical.' They make good company. ChristianKI says something important as well:

Without knowing the social environment in which you are operating it's hard to tell, but are you really sure you would lose all your friends.

You might be overestimating the probability that your tribe will abandon you. After all, that wouldn't be a very Christian thing to do, would it?

(More accurately, I'd label myself agnostic leaning atheist with regard to the existence of one or more intelligent world-designer(s), but I give almost no credence to any religious claims beyond that. In any case, for simplicity I'm just going to refer to myself here as an atheist.)

I used to say something really similar to this. I would say, "Nominally, I'm agnostic, but practically, I'm atheist." Then I thought about other, less important beliefs in which I could make a distinction between 'the nominal and the practical.' Say a person with whom I live leaves the house and goes to the store, and it has been some time, and another person asks where they are. Usually, I say, simply, "He is at the store." But this is not necessarily true. It is entirely possible that on the way to the store he was diverted from his usual route and out of kindness stopped to help a troubled motorist, and so he is nowhere near the store; or that the store has become the site of a hostage situation, and so no one may enter the store; or that the other has already, as we speak, been killed in a traffic accident, and so he may never enter a store again; etc. Yet, I do not tell the other resident, "Nominally, I am agnostic as to the whereabouts of our fellow resident, but practically, he is at the store." My veiled belief is that he is at the store, so this is how I act. It is undesirable to act contrary to this belief because the consequences are obvious and completely negative. It is easier with religious beliefs because the consequences are not as obvious and short-term positive (but long-term negative). Nominal beliefs are useless in life because life is always lived in practice. All to say, I have learned here that very little is certain and that that is far less important than one would initially think. Almost certainty is more than enough, and you and everyone else rely on that fact everyday.

My wife would probably decide to follow me - but there's a chance she might not, and I love her way too much to risk losing her.

I think it's funny (funny-strange, not funny-haha) that you say that you're not willing to risk losing her, but you go on for another paragraph after this about other reasons that you should not do this even if you are willing to risk losing her. It sounds to me like you, in fact, are willing to risk it, and rightly so in my opinion, and like that fact scares the shit out of you, and rightly so in my opinion.

Even if she did follow me it would cause her a tremendous amount of mental anguish which I really don't want to impose on her.

Realistically consider your ability to be exactly what is desirable to your wife for the rest of your life. Ask yourself if you think you can really avoid resenting her (and you do not have to be evil or lacking in character to be resentful) for the rest of your life. Never have I successfully willed myself to meet the expectations of others.

And that's besides the terrible emotional effects that a revelation of this sort would have on my parents, kids, siblings, and friends.

I really don't see how momentary 'grief' from the loss of a tribe member, even a community's worth, is worse than you feeling what you currently feel for a lifetime. And if you don't tell the kids, then you could perpetuate the cycle.

Talking to you is a moral hazard for me. I want to make more evangelizing atheists. I tried to be a counterpoint to your gloom more than an impartial advisor, and hopefully that resulted in a more thorough overview of the risks and payoffs of this decision. I say this because I see a lot of talk of the risks of coming out of the closet, but not a lot of talk about the payoffs, and when you do talk about them, you bury them in implications about risks. You're definitely continuing in a motivated fashion. Like everyone else, you also have an overwhelming compulsion to maintain the status quo.

One last piece of advice, since I see a lot of 'all about their feelings, and not mine': Learn that making sure that the rest of your life does not suck at the cost of some hurt feelings is totally okay, and that learning that will make the rest of your life not suck.