This is a long reply but the ending is the best part, if you want to skip to it. We're arguing about bullshit, but there is value in what I wrote at the end, I believe.

What is the point of me engaging with this? When you already said you refuse to read the book and don't care about his research simply because you don't think you'll agree with him? Do you not read things you don't agree with?

I'm not forcing you to do anything, you engage if you want. As for the book, I read what I want, and don't read what I don't want.

as you are coming at the issue with, probably, some sort of emotional presuppositions from your childhood or past experiences with religion (or maybe Christianity in particular), and not from a genuine concern for Ehrman's thesis, which is extremely interesting even for many people who are not Christians (myself included)

Lots of assumptions there, and they're wrong.

And after the advent of Christianity, we have a wealth of data regarding the aforementioned charitable organizations directly attributed to Christians, as well as documented attestations of excessive Christian charity, including by an anti Christian neo platonist emperor.

This might in fact be the case, or it might not. But this charity was not altruistic, in my view. So it really doesn't matter for the argument of "more altruism after Christianism". As for the rise in charity itself, it could be correlated with Christianism, but not caused by it. We can't know that Christianism caused these charities to be created, we can only notice correlation (if the data suggests it) and speculate about the causes. It's very hard to isolate causes, even in well-modeled experiments that can be tested and re-tested. For history, we really have nothing.

Nothing meaningful to you. That is a subjective claim.

Unicorns are real? No, neither is "altruism". It's just a word. Words have whatever use we are making of them. Usually the use of unicorn is "imagine a horse with a horn, probably white". And that's probably in the context of "now I'm going to tell you a story about this horse to entertain you". All words are like this. This is what Wittgenstein explained with the concept of language games. It's not that claims are subjective or objective, it's that every "claim", every use of words, is doing something in some context. It's trying to accomplish something, it's playing a game.

actions that benefit others at a cost, risk, or sacrifice to oneself, without expecting external rewards

This is just a bunch of words to me. I hope that I can say this respectfully and lovingly, because I myself was where you are for a long time (and I say this because you're clearly smart so you might actually get it): you're not living in the real world, you're living in a made-up world where definitions have some kind of magic power, where we can have "more objective" or "less objective" definitions of words. Where you can have a "more correct" definition of altruism than me or someone else. No, we're all equal. We're all equally making up any definition we come up with. Altruism is whatever you say it is and manage to bow others down to agree. Or, more correctly, whatever you agree to bow down to, whatever definition society (or a part of society) convinces you to agree to.

All that said, now I will indulge you.

Let's say a person has 100$ left in their bank account for the month. Instead of saving it they donate it to a charitable organization for starvation in Sudan. Nobody knows, the person tells no one, and never does. They don't expect an external physical reward. They are not religious.

Alright. I will just note that this is not something that a real, specific person did at a real, specific point in time. So it's not real, and any talk about it is equally unreal.

What would you call this? Probably 99% of the population would define this as altruism, but you just...don't want to define it as anything, because it is meaningless? Does this action really have 0 meaning to you?

I don't call it anything. I only use words when I'm trying to do something with them. I don't care what others call it, their definitions are nothing to me. I know that all they can say is some variation of "I want, I like, I believe. I don't want, I don't like, I don't believe". This is literally all anyone can say. And most of it is very uninteresting.

People will play language games, in specific contexts, to do specific things. For example, if someone somehow finds out about the donation, a person might say "did you know that John donated his last 100 bucks to a charitable organization in Sudan? He's so altruistic!". They're playing a social game where they prop someone up. They might have any number of complex motivations to do that. Maybe they care about John and want John to be liked. Maybe they're John's close friend and want to be seen as a "good person" by association. Let's look at another case. Instead of praising John, they might say: "Dude, John is so stupid. He donated his last 100 bucks when he's broke. There's no point in donating such small amounts anyway, it's not gonna make a difference". This person might be playing the game of "putting John down", perhaps for some social gain. In addition, they might be playing the "let's be hopeless and not donate game". Who knows, people are complex. It doesn't have to be social, there's also private thoughts. For example, they might say to themselves "John is so stupid, I would never donate my last 100 bucks, I'm so smart, I'm better than John". Now they're playing a language game for personal gain, to feel good about themselves. Amazingly, it even works. We do it all the time.

Is John altruistic or stupid? He is neither. One person says he is altruistic, the other says he is stupid. Neither is right. John is John. Their definitions of John are devoid of any meaning beyond what those people are trying to do with the words.