It's a false trilemma, as there are othe possibilities: he did not actually exist historically, or he was a wise, nice guy and others made up all the God stuff over the years, or he was a nice guy trying to help people and he thought tactically the mystical claims would allow him to help more people. One can probably imagine a bunch more or less realistic possibilities given a little while to think about it.
You must remember that the trilemma exists in a context. Lewis set it up as a reply to people who "say that they're ready to accept Jesus as a good human teacher, but not ready to accept him as God". That is, they already accepted that he existed, and that the set of teachings recorded was more or less accurate.
That rules out "did not actually exist historically" and "others made up all the God stuff over the years". (I mean, those exists as possibilities in an absolute sense, but not as possibilities that he had to answer, because the people he was answering didn't hold those as options.)
And to me, "a nice guy trying to help people and he thought tactically the mystical claims would allow him to help more people" sounds a lot like "liar". Liar with good intentions, but still liar.
Well, if you have to presuppose that the gospels are true then you are probably arguing with a tiny slice of people: those who believe the gospels are more or less accurate down to the specific quotes of Jesus but do not believe he was the son of God. I would say that "accept Jesus as a good human teacher" is a far broader category than this and nothing about it implies a particularly strict belief in the truth of the gospels.
I'd also say that, even accepting the gospels, we have to really take wide views of "liar, lunatic, or Lord" to make it work. Lewis himself further described the "liar" as akin to the "Devil of Hell" - I wouldn't characterize the liar I described as such. And he said the lunatic was akin to "the man who says he is a poached egg", but people are great at compartmentalization and can hold some very silly specific beliefs while still being otherwise rational, sane people. Even "Lord" admits plenty of things we wouldn't recognize as Christianity, e.g. rejecting the trinity or plenty of other heresies.
In the 1950s, in Great Britain, the basic accuracy of the gospels was a lot more widely shared assumption/presupposition than it is today.
If you're going to accept Jesus as a good human teacher, but not accept that he actually taught what the gospels say he did, then you have a good teacher but no teachings. And maybe that's a position that a lot of people like, because it leaves them an empty figure into which they can pour whatever teachings they personally favor, but I think it's rather a cop-out. (A steelman version would be: "I think he was a good human teacher, but no reliable record of what he actually taught survives, so we cannot claim that any particular teaching or position has his stamp of approval." But I still think that's weak. If you can't trust the record of what he taught, how can you trust the record that he was a good teacher?)
Accepting the gospels, the liar you describe is still offering people eternal life, at a price - "take up your cross and follow me". (The cross was not just a burden, it was an instrument of execution.) If the eternal life isn't there, it's hard to describe that as a benign or beneficial lie. Note that many of his followers were executed. (Our record of that doesn't depend on the gospels - Tacitus also says this.) You may say that this still falls short of Lewis's description, but it's far from your description. (I'd have to go back and reread Lewis's argument to see how tightly he has boxed people in on the "liar" branch.)
>Accepting the gospels, the liar you describe is still offering people eternal life, at a price - "take up your cross and follow me". (The cross was not just a burden, it was an instrument of execution.) If the eternal life isn't there, it's hard to describe that as a benign or beneficial lie.
If you don't actually believe in the supernatural stuff it seems like the definition of a benign lie. He thinks having society follow his broader teachings would result in a much better society on net, regardless of some people being killed for following and spreading it. He thinks that you go in the dirt when you die so he says, hey, actually you go up to this great place. This really helps it catch on and spread. Nobody finds out that part's not true because when they die they just go in the dirt.
Any kind of large-scale change will have some specific people dying compared to the counterfactual of not making the change. To assess whether the change is benign, positive, or negative, we must grapple with both the positive and negative impacts.
For example, I could wave a magic wand today and fix climate change, saving millions or billions of lives. But since I did so, some granny slips on a patch of ice next winter and dies from the fall. Without my magic wand, she would have lived.
Now, I don't specifically know who this granny is before I choose to wave my wand, but as a thinking individual I understand that some such granny must exist. In fact, there are probably many grannies who will die as a result. Would you argue that waving my wand is not a positive change because specific people exist who will be harmed?
I don't think it rules out "others made up all the God stuff over the years". It's perfectly reasonable to take John with a big pinch of salt and believe that Jesus did not claim divinity, and I wouldn't be surprised if Lewis' intended audience included such people.
It's highly debatable whether Jesus believed himself to be divine. There are more than three options, for example Jesus may have simply made a mistake in his own reasoning. It is not as inconceivable as Lewis makes out that Jesus was a lunatic or a liar.
