So now let's talk about Jesus.
A couple of people on Patheos, including Bob Seidensticker and Leah Libresco are talking about a "tetralemma" version of Lewis' famous argument. Since Christ claimed to be God (Lewis says), either he was right, he was a dirty self-aggrandizing liar, or he was insane "on the level with the man who says he is a poached egg". Atheist writers have noted a fourth possibility: that he didn't exist at all.
This, then, is the famous "lord, liar, lunatic, legend" argument. The modern conception of Christ as "just a wise moral teacher" is preposterous - wise moral teachers don't claim divinity if they're not divine. Most moderns don't want to go full mythicist and say Jesus didn't exist at all. But most moderns also say believe his teachings were too profound to be the work of a total charlatan or a raving lunatic. Therefore, Christ was God.
It's a lot like how we are forced to either admit Kary Mullis was really abducted by raccoons from outer space, or else reject the entire field of genetics as a monstrous sham.
I am not an expert on lords or liars, but I have worked at some psychiatric hospitals and known a couple of lunatics in my time. Some of them are exactly what you would expect - extremely low-functioning people who are bouncing off the walls and smearing themselves with feces and probably not fit for polite society. Others are not quite so stereotypical. Kary Mullis wouldn't make it into a psych hospital - his weird hallucinations don't impair his ability to function or make him dangerous to himself or others - but I think it would be fair to classify him as having some kind of subclinical psychological issues. But these don't impair his ability to do some really brilliant chemistry.
The diagnosis of delusional disorder technically requires "nonbizarre" delusions - things that could in theory happen, like that a friend is plotting against you, as opposed to things that could not happen, like that you can fly. But psychiatrists naturally have looser standards for "nonbizarre" than the general population, and God talking to you or choosing you in some way seems like a pretty accepted textbook case.
One of the most notable signs of delusional disorder is that "apart from the delusion's ramifications, functioning is not markedly impaired." The Corsini Encyclopedia of Psychology admits that "the rates of marriage and productive vocational functioning in delusional disorder are generally within the expected range" - it adds that there is some evidence for some impaired functioning, but it doesn't provide the evidence and the impaired functioning isn't necessarily present in all cases.
Most sources describing delusional disorder are impressed both by the complexity of the delusions ("often highly detailed and may remain unchanged for years") and by the relative sparing of logic, including the logic with which the delusions are implemented. If a patient believes they've discovered an alien invasion plot, well, that's crazy, but they will act in pretty much the same way anyone who did discover an alien invasion plot would act: internally debate whether to conceal it so they don't look crazy, or to try to contact and convince the President so he can prepare.
(the psychiatric hospital where I interviewed in DC had what they called the "White House Ward" for delusional patients from all over the country who had come to Washington to convince the President of things.)
The DSM-IV-TR lists as one common subtype of delusional disorder "delusions of...special relationship to a deity". So you can see where I'm going with this. Modern psychiatry believes it is totally possible to believe you are the Son of God while keeping your reason and sanity in every other area of life. It wouldn't even be considered unusual. If you were a Nobel Prize winning chemist before the disorder presented, you would become a Nobel Prize winning chemist who delusionally believed he was the Son of God. And if you were a great moral philosopher before, you would become a great moral philosopher who delusionally believed he was the Son of God.
But I'm not sure this is the whole story. A lot of my views on religion changed after reading Aleister Crowley's Book Four, where he argues that the founding of religions depends on mystical experiences that occur predictably and regularly if you take the right steps (fasting, prayer, meditation) as a consequence of currently-unknown psychological or neurological factors. Crowley describes one such experience as:
...usually comes as a tremendous shock. It is indescribable even by the masters of language; and it is therefore not surprising that semi-educated stutterers wallow in oceans of gush. All the poetic faculties and all the emotional faculties are thrown into a sort of ecstasy by an occurrence which overthrows the mind, and makes the rest of life seem absolutely worthless in comparison...also the conditions of thought, time, and space are abolished. It is impossible to explain what this really means: only experience can furnish you with apprehension.
A further development is the appearance of the Form which has been universally described as human; although the persons describing it proceed to add a great number of details which are not human at all. This particular appearance is usually assumed to be "God." But, whatever it may be, the result on the mind of the student is tremendous; all his thoughts are pushed to their greatest development. He sincerely believes that they have the divine sanction; perhaps he even supposes that they emanate from this "God." He goes back into the world armed with this intense conviction and authority. He proclaims his ideas without the restraint which is imposed upon most persons by doubt, modesty, and diffidence.
I don't know if I would describe this as "insanity"; it would be pretty mean to the mystics. But I can certainly see it as using some of the same psychological certainty as delusional disorder: this one blast of psychic energy embeds an idea into the deepest layer of the brain while mostly sparing everything else. Just as psychoactive drugs can mimic or cause psychiatric disorders, so this natural high from meditation and self-cultivation might have some of the same effects. The Bible tells us that Jesus fasted in the desert for forty days. And since that seems like an odd thing to do out of the blue, maybe he spent some of his eighteen "lost years" hanging out with "fast-in-the-desert" sorts of people like the Essenes
So we find that this third horn of the argument, "lunatic", is a lot more complicated than Lewis gives it credit for. There are lunatics who appear perfectly sane in every way but one, and that one may be a preoccupation with being a deity or the relative of a deity. And it may be that sufficient religious preoccupation can artificially bring on this form of unshakeable belief. To me, this is a much more likely version of Jesus than a liar, a garden-variety lunatic or the one person who thought he was the son of God and was right.