We now have undeniable proof that one of your core beliefs is objectively wrong. Which one do you think it is and what is the new difficult truth for you to swallow?

That people are fundamentally benevolent to one another. Obviously it can be trained out of you by circumstance, overcome by self-interest, and mental illness is a thing, but I think people innately care for one another. It's why dehumanization is the first step to committing atrocities.
But if someone offered proof that I'm wrong that might be the least surprising thing that happened all week. And if I'm wrong, the evil-doers are sub-human and should be culled without mercy until I am right.
- The evil-doers are sub-human and should be culled without mercy until I'm right.
  I know what you mean but that sentence is really funny when 1.5 sentences earlier you said "it's why dehumanization is the first step to commiting atrocities" haha
  
  It's the intolerance paradox in action. It's like tolerating cancer. Cancer is a living thing, it doesn't mean you respect it and let it have its way with you without interference. Same principle.
  
  It was intentional irony.

That we can build a sane, rational society.
- One could argue by historical standards that we're closer than not already. How sane would you say sane is?
  
  I thought that Western style Democratic republics were leading the world toward purely secular forms of government, but yet another group of sociopaths has managed to take power. They have distracted the science-illiterate majority into petty conflicts based on different versions of magical thinking.
  So, "sane" would mean that we don't elevate the least sane among us (sociopaths) into positions of power. "Rational" would mean that public policy decisions are mostly made based on evidence, rather than fundamentally irrational belief systems.
  I fear that we are barely-sentient primates doomed to repeat the same awful mistakes, when simple, obvious solutions are within our grasp.

That people's 'default' morality is 'good'.
It isn't. It is actually pure apathy and only do we get taught, groomed, learn, decide, etc. about morality.
If that is true, then some people are actually 'better' and 'worse' than others. If so, then my entire outlook on human life will need to change. Don't know to what, but that is the existential threat.
Recently had to come to the conclusion, that even though I have never 'tried' to learn, observe, or otherwise be smart, that I am well above average intelligence to those immediately around me. This is beyond infuriating. How can I be 'better' than everyone on average without even trying? It infuriates me to no end.

That people can change through conversations. It’s tough to accept, but most people only change when forced to.
- I've noticed 2 types on this, stick-in-the-muds and peak-hunters.
  Stick in the muds latch on to the first version of a belief they encounter properly. They will stubbornly hang on to that for as long as possible.
  Peak hunters are the opposite, they will rapidly change beliefs to maximise the results/find truth.
  Interestingly, after some time, the 2 groups look almost identical. The peak hunters tend to find the 'best' version of their belief, based on their existing memeplex. To budge them, you need to show a different belief is better, on their rankings (not yours). This is hard when they have already maximised it. Without knowing how they are weighing things, they can look like stick in the muds.
  The biggest tell is to question why they believe what they do. If they have a reasonably comprehensive answer, they are likely peak hunters. Stick in the muds generally can't articulate why their belief is better, outside of common sound bites.
  
  I understood this and think it’s accurate.
  
  What do you think the ratio is there? And how much does it vary culture to culture?
- People or beliefs?
  I've changed my mind many times based on online discussions.
  
  Beliefs. I’ve changed my mind too, but it seems to be the exception.

That people are not wilfully stupid. The last 10 years have proved people will act against their own benefit if TV tells them to do it.

That climate change won’t wipe out humanity. I firmly believe we’ll survive, but it will be a massively devastating event, like 1/3 of the population will die. I think the equator will probably become uninhabitable, but more northern or southern land will become more like the equator. Maybe I’m wrong though, and we won’t survive. Maybe there’s a reason we don’t see any advanced space faring civilizations.
- I didn’t mean to make it sound instant, but I don’t think it’s going to take hundreds of years. I think it’s more on the order of decades. The deaths I’m talking about will come from things like floods, famines, hurricanes, heat waves, etc.
  
  Huge migrations
  And you think the ensuing wars alone wouldn't kill half of us? It's going to be a shit show that the youngest generation will get to experience in its fullest
  
  New and fun communicable diseases has entered the chat.

I can't think of any that I'd be particularly surprised by at this point.

Probably the belief that there (isn't) some kind of omnipotent god interested in guiding our affairs. It's not like I'd ever be able to know. How would I cope? Pretty easily. It'd be comforting. That's a pretty good reason to doubt it, since I'm biased in favor of it.
The thing is, I'm not picking this one because it's the most likely one, but because all of the other "core" beliefs are either completely subjective judgments that can't be "wrong" or are flexible enough that it doesn't really matter.
- And omnibenevolent? A god that's just screwing with us is feeling relatively more likely these days, and I'm not comforted.
  
  that part is tautological. if the almighty is perfection, then benevolence is defined by it, not by us. so the only way one can consistently hold that belief is to admit that we are the imperfection. we are the disease, we are punished. i guess that's why i dont believe that shit lol

"Bullshit. There's no way the moon is actually made of cheese. That's just a stupid story you tell kids!"

That there's no such thing as too much inclusivity in LGBTQ.
I don't think people who want to pretend to be dogs or cars or whatever inanimate object they fixated on as a child are harmful to society, but they have proven to only delegitamize actually real gender identities that are being actively erased in the real world.
I don't care if people want to wear collars and shit in litter boxes because that makes their brains happy, but I do care when those people show up in public places wanting to be treated with the same seriousness as actually marginalized minorities and get LGBTQ movements laughed out of the room.
- I tend to agree with you, but this same line of thought is used by many in the queer community to 'other' bisexuals and trans people, for example. Everyone draws the line in a slightly different place. I don't know what the right answer is. For me I would probably draw the line between 'sexuality' and 'fetish'. Your sexuality should be protected from discrimination and persecution, but in my mind a fetish is more akin to a hobby or sport you enjoy and wouldn't deserve the same level of protections or attention.
  You can easily choose not to walk around in a dog collar on a leash in a rubber suit in public, because you're just doing it for kicks. You can't choose not to be queer.

The cream does, in fact, rise to the top. All the rich idiots I've met were brilliant in a subtle way I couldn't appreciate, and I myself am just being lead on by the people I know about having redeeming qualities of any kind. All the studies showing social mobility is small both upwards and downwards had some kind of fatal flaw built into their assumptions.
Stuff about moral decline is another candidate. In both cases, I'm actually pretty sure everyone in every time, place and walk of life has roughly the same capacities on average, but narratives suggesting otherwise are so damn pervasive I have to wonder if I'm missing something.

That all living things are worthy of my compassion. If the millions of conservatives out there somehow prove me wrong... then all attempts at civilization are doomed to collapse and we're reverting back to feudal times.

Would be interested in a list of past facts that turned out to be wrong.
- Government of the people, for the people, by the people.
- The earth is flat.
- That the sun is the center of the universe

The way I landed on all my current beliefs was taking in information from as many places as I could and I decided on what I think is right.
There are a ton of topics that can't have an objectively correct answer which makes things fairly complicated.

I don't think it could be anything I expect. Most of the things I have consciously evaluated about myself I've come to a conclusion based on rational or empirical evidence, so I am certain either in my knowledge or ignorance about a topic. Most of the time when I've been proven wrong it's about a belief I imbibed as a child and never questioned or considered until then.

I'm actually good at what I do, and everyone actually likes me and doesn't think I'm just dead weight.

My deepest core belief is that there is a non-zero likelihood (which may be quite high) that everything I think I know about the world is wrong.
If it was proven to me beyond a doubt that something I know is undoubtedly correct, I would probably think that there was a possibility that the proof was wrong and go on with my day.
- The world is a simulation. Everyone you see is just an AI construct. WAKE UP
  _/s
  
  Why try waking them? The system's just going to repair itself. They go on believing the TRUTH is false.
  
  Relevant user name?
  
  Why, oh why didn’t I take the blue pill?

That I live in a constitutional democratic republic

For me it would be that while lies are in many cases morally justifiable.
My current belief on this is that lying is never right unless you're literally using it as a form of self defence as an alternative to physical violence. However, I also tend to believe that absolute beliefs are virtually always wrong, and these two are conflicting beliefs. I can atleast think of a few extreme scenarios where a white lie seems justifiable even when you're not in danger. For example: a dying person showing their painting and you complimenting it despite not liking it.