I have had recent occasion to reference C. S. Lewis and his famous “lunatic, liar, or Lord” gambit. Lewis presents an argument in the form of a “trilemma” (this being a choice between three unpalatable options—a dilemma plus one). He addresses his argument to those many non-Christians who maintain that Christ was a “great moral teacher,” though not God. He attempts to show that this position is contradictory.
In brief: Christ himself claimed to be God. So if he is not God, he is either lying (knowing he isn’t God but saying he is) or crazy (thinking he is God when he is not). In either case, he cannot be a “great moral teacher”—for a liar isn’t “moral” and a lunatic isn’t “great.” The only way out is to accept that Christ is the God he said he was.
Criticism of Lewis usually goes the path of denying (i.e., attacking the premise) that Christ claimed to be divine. This works best when it comes to Christ’s more explicit (supposed) claims to divinity; some of these are almost certainly mistranslated or interpolated by Christians after the fact . It is harder to discount Christ’s more implicit claims to Godhood—say, in his forgiving sins committed against third parties (something, Lewis points out, can only be done by the third parties or by God himself.)
Other critics attack the logic of the argument. Some deny that Christ existed, even as a historical personage, expanding the trilemma into a “quadrilemma”: “Lunatic, liar, Lord, or myth.” (Some of Lewis’s defenders accept this posing of the problem, admitting Christ’s nonexistence as a logical option while denying the truth of that option.) This tack has appeal and is probably my own view; philosopher Michael Martin mounts a vigorous, independent defense of the “Christ as myth” view in chapter one of his “The Case Against Christianity.”
But it isn’t necessary to believe Christ never existed to diminish the force of the trilemma. To the “myth” angle I would add a few of my own, which don’t depend on such a “radical” conclusion:
(1) We might expand even further to a quintilemma: Christ could have simply been mistaken about being Divine.
The “simple” here is relative: While it may seem far-fetched that a (sane) person could think himself God, this is inexpressibly less far-fetched than that there could be such a thing as a God. And this is less far-fetched still than that there could be a man that is also God. (A man by definition cannot be God, just as he cannot by definition be a duck or piece of twine; what it means to be a man rules out these possibilities on logical grounds.)
Indeed, there is probably no delusion that it is impossible for a sane person to maintain. For “lunacy” is not defined by a single fixed, false belief; nor is one such belief sufficient evidence for a “lunacy” diagnosis.
(2) On the other hand, it is simply unwarranted to assume that having a mental illness, a delusion, is incompatible with being a great moral teacher. Certainly, we know of “great teachers” of subjects other than morals—John Forbes Nash of mathematics fame, for instance—that suffered serious fixed delusions while doing the very work that makes them “great.” Why should morals be any different in principle? (Again, we don’t have to think a “lunatic” moral teacher is very likely—just that it is easier to believe in than a man-God.)
(3) The notion that Christ is a “great moral teacher” is also suspect. First, two points to keep in mind:
The burden of proof for “greatness” must be very high. We cannot mean “great” in the sense that our uncle is a “great” chef or our own fifth grade teacher was “great” with kids. Christ is a moral teacher for the ages; he is historically great.
Second, I am assuming Lewis’s “great” does not mean “influential,” “lasting,” or any other term which refers to the effect or reception of Christ’s teaching rather than the content of the teaching itself. For surely, though a popular moral teacher can be “great”—and part of what makes him great accounts for his popularity—it is not popularity which makes him great. We have to assess what he says, and how he says it. A great moral teacher will have great moral teachings. So “great” will mean “profound,” or something like that.
This in mind, it is doubtful Christ’s teachings qualify. The Golden Rule, from what I can tell, has an analogue in every other religion, including the Judaism Christ ripped it from. If teaching the tenets of one’s own religion makes Christ “great,” every Christian who teaches it after him is “great” as well.
And the Rule isn’t so much an ethical point, as a presupposition of doing any ethics at all: It is a call to put ourselves in the other’s place—to be empathetic. But we do ethics to find out how to be empathetic, not to know to be empathetic; you already have to know to be empathetic to be interested in doing ethics in the first place! A morals teacher asking his pupils to empathize is no more profound than asking them to “be good,” or to “do the right thing.”
Overall, Christ’s body of teaching is not particularly rich. It mostly takes the form of injunctions or “points” rather than arguments. There is nothing like a “system” of thought. There are statements but not much reasoning among the statements. We aren’t told why we should do what he says to do. When you place this alongside the work of a “great moral teacher” like Aristotle, who predates Christ, with his ethical literature, its comprehension and detail, there is no comparison. He offers a methodology for knowing when something is right or wrong, organizes the virtues in a broad taxonomy, and situates it all within an interdisciplinary system of thought. If both Christ and Aristotle are “great,” the spectrum of “greatness” is so top-heavy as to be unusable.
(4) Finally, the same trilemma can be made of other figures and placed before Lewis and the Christians themselves. Plenty of persons other than Christ have claimed to be God. Some of these offer no more evidence of “lunacy” than Christ himself; and of these, some are religious leaders who hold as much claim to “great moral teacher” as Christ. That is, if Christ can “pass” as sane and “great” and honest, so can these. By Lewis’s reasoning, then, all these men and women are God. He is polytheistic, or his reasoning is bad. (Of course, his reasoning is bad.)
 See theologian John Hick’s “The Metaphor of Christ Incarnate.”
 Lewis’s contention that a mere man who thinks he is God is “on the level with the man who says he is a poached egg” draws a poor analogy. For one, the belief that one is God is at least a coherent one (at least, as coherent as the idea that a man, some man, could be God). But the belief that one is a poached egg is very unlike this. The very idea is contradictory: Since a poached egg cannot have thoughts, a man who is one could not have the thought that he is one. (Therefore, if he has the thought that he is a poached egg, he cannot be a poached egg.) The belief carries within itself the means of its own discrediting. Unlike the belief in one’s own Divinity—which can be validly held by at least someone (namely, the one true God)—one cannot believe oneself to be a poached egg without holding a lot of other fucked up (crazy?) beliefs at the same time (beliefs about the nature of objects, etc.)