Clive Staples Lewis (1898-1963), usually known as C. S. Lewis ("Jack" to his friends), was an Irish-born British writer, scholar of English medieval and renaissance literature, and Christian apologist. He was a fellow of University of Oxford from 1925 to 1954, the first Professor of Medieval and Renaissance Literature at University of Cambridge from 1954 until his death.
While some religious apology is magnificent in its limited way — one might cite Pascal — and to some it is dreary and absurd–here one cannot avoid naming C. S. Lewis — both styles have something in common, namely the appalling load of strain they have to bear. How much effort it takes to affirm the incredible.(Christopher Hitchens), God Is Not Great
The Lewis Trilemma is an apologetical argument for the divinity of Jesus, invented c. 1844 by preacher Mark Hopkins (published 1846 in Lectures on the Evidences of Christianity) and popularized by C.S. Lewis on BBC radio, hence the name of the trilemma. The argument is also known as "liar, lunatic, or Lord," or "myth, madman, or messiah" referring to the three given parts of the trilemma.
Lewis's own statement of the argument
Lewis wrote in Mere Christianity:
I am trying here to prevent anyone saying the really foolish thing that people often say about Him: I’m ready to accept Jesus as a great moral teacher, but I don’t accept his claim to be God. That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic — on the level with the man who says he is a poached egg — or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a madman or something worse. You can shut him up for a fool, you can spit at him and kill him as a demon or you can fall at his feet and call him Lord and God, but let us not come with any patronising nonsense about his being a great human teacher. He has not left that open to us. He did not intend to. ...Now it seems to me obvious that He was neither a lunatic nor a fiend: and consequently, however strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God.
Note that his own words appear to undermine premise 3 of his argument below, which relies on Jesus being a great moral teacher to rule out the possibility of his being a lunatic. Jesus, if he existed may have been mistaken over being God but rational enough to know he wasn't a poached egg. Also it's not clear Jesus said he was God. The Gospels were written decades after Jesus and what Jesus actually said may have been different from what's in the Gospels.
The plausible natural explanation for the Jesus stories is that they were told orally for decades, and they grew with the retelling, changing to fulfill prophecy from the Law or to ensure that Jesus took on the traits of competing religions. Remember that Palestine was the crossroads of Greek, Egyptian, and Mesopotamian cultures. 
Also it was a few centuries after the time of Jesus when the Gospels were formally established in the form we read them today. What we find in the Gospels has as much to do with what early Christians believed as with what Jesus (if he existed) actually said. Romans formalized the New Testament, decided what to keep and what to leave out. Therefore what we find in the Bible has a great deal to do with what early Roman Christians believed or wanted to believe and does not only reflect first century Judea.
The logic of the argument
This can be summed up as:
1. Due to the law of the excluded middle, Jesus was either correct or incorrect in his claim of divinity. He also either believed or did not believe he was divine. Therefore, Jesus either:
- 1A. Believed he was God and was correct in his belief (Lord).
- 1B. Believed he was God and was incorrect (lunatic).
- 1C. Did not believe he was God and was correct (liar).
- 1D. Did not believe he was God and was incorrect (ignored).
2. Jesus spoke out against liars, so he was not a liar.
3. Jesus did not show any other signs of being deranged, being a great moral teacher, so he was not a lunatic.
4. Therefore, Jesus was God.
Problems with the argument
The above argument holds a few hidden premises beyond the ones given outright by Lewis. These hidden premises include:
- The Abrahamic God exists.
- Jesus existed.
- The Gospels are an accurate record of Jesus' life and teachings.
- Hypocrisy is impossible.
All of these hidden premises appear when we formalise the trilemma, and can all be challenged or undermined, with the last one breaking the logic alltogether. Anyone with any non-zero amount of social activity has tons of experience of hypocrisy in their lifetimes. Never mind that the very statement "hypocrisy is impossible" can be considered blatantly hypocritical. Even young children can remember cases where they or someone else was hypocritical.
There are also more problems with the argument beyond these alone. Step 4 in the argument can be challenged, as Jesus may have had that one delusion of false belief of auto-divinity but been otherwise okay, much like how Isaac Newton was an alchemist and tried to find messages in the Bible but otherwise made great contributions to science.
One could disagree with the claim of Jesus being a great moral teacher. A great deal of Christian morality is about inducing guilt and reducing self-respect. The teachings of Jesus have been used to justify widely divergent belief systems like Christian communism, Christian economics and Liberation theology. The teachings of Jesus were used to justify the Spanish Inquisition and various other persecutions of those considered heretical by a range of different Christian orthodoxies. How the moral teachings of Jesus are interpreted has a great deal to do with how later Christians developed what is in the New Testament.
In Lewis' specific version of the trilemma, it is not apparent how Jesus would not have been a great moral teacher simply because he falsely believed himself to be God and for no other reason. This blends with the above paragraph: Newton practiced the pseudoscience of alchemy, yet that did not make his contributions to actual science any less valuable.
The true essence of the argument is that Lewis, in reading the words attributed to Jesus in the Gospels, simply had the subjective opinion that it did not seem like they could have come from anyone but God, and the rest of the argument is mere window dressing around that core.
Our fourth, ignored possibility - of Jesus not knowing he is God - also shows up. It isn't enough to refute the trilemma, beyond turning it into a "tetralemma," and is little more than a trivium due to the confusing implications which nobody, either theist or atheist, has really explored in depth. Anyhow, that possibility isn't part of the established story and no one has made the claim (that we know of). Beyond that, Jesus claims his own relation to divinity multiple times throughout the Bible.
A version of the trilemma appears in his fiction book The Lion, the Witch, and the Wardrobe in the Narnia series. After Lucy visits Narnia and tells the other three children, they dismiss her story. The owner of the house at which they are staying, however, argues that since Lucy is neither dishonest nor insane, her story must be true. This shows both the absurdity of the argument, and the disingenuity in employing it (the man had already visited Narnia, and thus this argument wasn't his true reason for believing Lucy, just as Christians have other reasons for believing in Christianity).
Lewis gets Hitchslapped
Christopher Hitchens takes up the old coot on his trilemma, shedding the following light in his (itself rather great) God Is Not Great;
There were many deranged prophets roaming Israel at the time, but this one reportedly believed himself, at least some of the time, to be God or the son of God. And that has made all the difference. Make just two assumptions, that he believed this and that he also promised his followers that he would reveal his kingdom before they came to the end of their own lives, and all but one or two of his gnomic remarks make some kind of sense. This point was never put more frankly than by C. S. Lewis (who has recently reemerged as the most popular Christian apologist) in his Mere Christianity. He happens to be speaking about the claim of Jesus to take sins on himself: Now, unless the speaker is God, this is really so preposterous as to be comic.
We can all understand how a man forgives offenses against himself. You tread on my toes and I forgive you, you steal my money and I forgive you. But what should we make of a man, himself unrobbed and untrodden-on, who announced that he forgave you for treading on other men’s toes and stealing other men’s money? Asinine fatuity is the kindest description we should give of his conduct. Yet this is what Jesus did. He told people that their sins were forgiven, and never waited to consult all the other people whom their sins had undoubtedly injured. He unhesitatingly behaved as if He was the party chiefly concerned, the person chiefly offended in all offenses. This makes sense only if he really was the God whose laws are broken and whose love is wounded in every sin. In the mouth of any speaker who is not God, these words would imply what I can only regard as a silliness and conceit unrivalled by any other character in history. It will be noticed that Lewis assumes on no firm evidence whatever that Jesus actually was a “character in history,” but let that pass. He deserves some credit for accepting the logic and morality of what he has just stated.
To those who argue that Jesus may have been a great moral teacher without being divine (of whom the deist Thomas Jefferson incidentally claimed to be one), Lewis has this stinging riposte: That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on a level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman and something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to.
I am not choosing a straw man here: Lewis is the main chosen propaganda vehicle for Christianity in our time. And nor am I accepting his rather wild supernatural categories, such as devil and demon. Least of all do I accept his reasoning, which is so pathetic as to defy description and which takes his two false alternatives as exclusive antitheses, and then uses them to fashion a crude non sequitur (“Now it seems to me obvious that He was neither a lunatic nor a fiend: and consequently, however strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God.”). However, I do credit him with honesty and with some courage. Either the Gospels are in some sense literal truth, or the whole thing is essentially a fraud and perhaps an immoral one at that.
Well, it can be stated with certainty, and on their own evidence, that the Gospels are most certainly not literal truth. This means that many of the “sayings” and teachings of Jesus are hearsay upon hearsay upon hearsay, which helps explain their garbled and contradictory nature. The most glaring of these, at least in retrospect and certainly from the believers’ point of view, concern the imminence of his second coming and his complete indifference to the founding of any temporal church. The logia or reported speeches are repeatedly cited, by bishops of the early church who wished that they had been present at the time but were not, as eagerly solicited thirdhand commentaries.
- I.e., a liar.
- C.S. Lewis Gets it Wrong: Liar, Lunatic, Lord … or Legend?
- I.e., everybody over age three except for good ol' Jack, whose literature indicates he has not talked to any actual humans for years.
- Christian Morality: Hostile to the Individual and Society
- The Myth of Christian Morality
- Just a dilemma with four conflicting possibilities, much like a trilemma has three or an ordinary dilemma two.
- Christopher Hitchens, God Is Not Great: How Religion Poisons Everything (p. 118-120)