First, for Lewis' rendition of it:
"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."Now, there are a few good objections to this already presented in the wikipedia article itself. Namely, that Jesus could be fictional, that the Gospel could be inaccurate, that Jesus did not necessarily refer to himself as the "Son of God" and he was only claimed to be after the fact by followers, and that the argument isn't logically sound (hence, the previous alternatives to the "trilemma").
But, really, the argument he makes, even barring that fact, is completely and purposefully ignorant of human behavior. A "lunatic" is hardly crazy about everything. People who have such a delusion can still have insight. Hell, if you have the right kind of delusions, you might not even be called "crazy". It is not as if someone can hold one bizarre, counterfactual idea and suddenly anything that comes out of their mouth is suspect. Think about it for a brief moment. Is it impossible for a Christian, even if they KNOW their worldview is true, to acknowledge Buddha as a good moral teacher, even though he believed in reincarnation? If David Berkowitz said that you shouldn't punch schoolchildren in the kidneys, can we not judge that idea on its own merits, or should we automatically dismiss it on the basis that it was said by a mad serial killer who thought that his neighbor's dog was talking to him? Even those with questionable beliefs, or who actually have severe psychological problems that would cause one to be called a "lunatic" can say things that make sense. And, considering that there are very few people who are completely insane, I feel that this particular line of thought was a bit of a shell game. I'll clarify in a bit.
As for saying that he was a fiend (or, I assume, a malicious liar), I can't offer anymore of a rebuttal than to say that he was perhaps just an innocuous liar whose believers took the bluff a little too far. I don't understand what about his potential lies would cause him to be seen as utterly depraved and demonic, especially if he was human enough to not see any negative implications.
Now, a brief look at the formalized version of the argument:
The premises are as follows.
- (P): Jesus claimed to be God.
- (Q): One of the following must be true.
- Lunatic: Jesus was not God, but he mistakenly believed that he was.
- Liar: Jesus was not God, and he knew it, but he said so anyway.
- Lord: Jesus is God.
From these premises it follows logically that,
- (C): If not God, Jesus is either not great or not moral
As mentioned above, Premise P is suspect, and Q should probably have other options. The typical method of getting to the Conclusion as by ruling out option 1 by claiming that an insane person would not have his kind of insight. Funny, though. Because the definition used simply says that "he mistakenly believed that he was [God]". That isn't so much "insane" as, I dunno, "mistaken". A guy with human fallibility, grasping for a big purpose by playing around with the tales his mother told him about his "virgin" birth in his head for a while.
He is typically proven to not be a liar because he would be evil otherwise, and he is not shown to be evil in the Bible. But, of course, humans are anything but consistent. And we all have our vices of choice. Might just be that Jesus had a taste for being a charlatan with a penchant for rhetoric, rather than being a sex maniac, petty thief, or axe murderer. Maybe different kinds of "evil" don't exactly translate over. Or, maybe it was all just a metaphor...
Bottom line: You can be good and still lie. You can lie, and still look good. You can look crazy to some, and seem profound to others. And even a nutcase when it comes to one subject can be right when it comes to another.
This argument ignores that entirely, changing a "bluff" into a "malevolent scheme" and "delusions" into "complete and utter madness seldom seen in actual human beings outside of fiction" in order to disprove the much stronger, much more pronounced distortion of the original idea, rather than the more accurate, less refutable possibilities. That, in addition to the other shortcomings that have been amply covered elsewhere, makes it very weak in terms of anything short of adding to the confidence to those who do not wish to see the argument's faults.