There is no need to go as hypothetical as that. There are plenty of routine small-scale "emotional basilisks" in everyday life, like whether to finally tell your partner that you had a one-time fling while on a business trip many years ago.

If you want to consider a hypothetical, you might not want to introduce terms and subjects seen to conflict with the hypothetical. I think that this post would have gotten a somewhat better response if you just replaced "atheism" with "X". The handle "atheism" is just distracting at this point in the discussion.

If I could, I would work out a way to tell people that would cause a minimum amount of societal disruption, but I would definitely tell people. Since I do not and cannot know what the future holds, I cannot risk future generations making bad decisions based on false information. Since I respect other people as adults with the right to make their own decisions, I do not have the moral authority to decide it would be in their "best interests" to be lied to (as some parents do with their children).

While I don't entirely think this article was brilliant, it seems to be getting downvoted in excess of what seems appropriate. Not entirely sure why that is, although a bad choice of example probably helped push it along.

To answer the main question: need more information. I mean, it depends on the degree to which the negative effects happen, and the degree to which it seems this new belief will be likely to have major positive impacts on decision-making in various situations. I would, assuming I'm competent and motivated enough, create a secret society which generally kept the secret but spread it to all of the world's best and brightest, particularly in fields where knowing the secret would be vital to real success. I would also potentially offer a public face of the organization, where the secret is openly offered to any willing to take on the observed penalties in exchange for the observed gains. It could only be given out to those trusted not to tell, of course, but it should still be publicly offered; science needs to know, even if not every scientist needs to know.

There's nothing intrinsically bad about being wrong, but it's risky. At the very least, you should probably tell powerful people. The cost of making them less happy is the same as with anyone else, but the benefit should accurate knowledge matter is much greater.

Sometimes one not only has to fight hypotheticals, one has to fight reality. Even if I consider the hypothetical situation as is, the person in that situation should look for ways to overcome that existentialism and doubt. For a real-world example, Eliezer certainly isn't troubled by it, and has set out at length why one shouldn't be.

I'm a newbie.

I'm generally sympathetic to the pragmatic benefit over "scientism" (i.e., ignoring actual empirical effects based on what simply must or couldn't be the case) view. (In fact, I plan to post a little piece on that matter shortly.) I'm a fan of the so-called "tetris effect" of learning how to be happy by doing/noticing.

I make a distinction in the case of theism/atheism for two reasons, though. First, not to put to fine a point on it, theism is too wacky/undefined. Religious people tend not to really have any idea what the hell they are asserting when they say they believe in God or providence or whatever. An "omnipresent force," "a benevolent watcher," karma, Jesus, Jesus's old man, Vishnu, the tao, a first cause--who the hell knows what they are talking about. Second, I think you can get the same psychological benefit (which I don't doubt) without any of the cloud cuckoo land.

What used to be called "positive psychology" got a bad name around the time of Freud because it became increasingly clear that it in't as successful at helping people with serious mental problems ("hysteria") as drugs, the talking cure, shock therapy, etc. What it is good at is increasing (a bit) the the quotient of general happiness in the practicer. And the thing is, religious practice isn't so hot at fixing psychosis either. They're about on a par. But utilizing the "tetris effect" doesn't require one to believe anything...well...stupid. It's just a practice, like meditation, that makes one feel better.

In sum, my feeling is that there's no harm in getting these benefits--even if they're something of a Pollyanna/placebo thing. But all else equal, if you can get such benefit without the use of bronze age fairy tales, that's the route that makes more sense. If your hypothetical were adjusted slightly to insist that there is no other way to receive the longevity and happiness provided by "theism" (whatever that is, exactly), I might revise my answer. But I think I'd still want to know precisely what bilge I'd have to swallow to get this increase in utility.

W