The following two quotations, from Lewis’ Mere Christianity, constitute Lewis’ well known lunatic, lord, or liar argument, sometimes called Lewis’ “trilemma” or “mad, bad, or God.”
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 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 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.
We are faced, then, with a frightening alternative. This man we are talking about either was (and is) just what He said or else a lunatic, or something worse.
C.S. Lewis, Mere Christianity, book 2, chapter 3, paragraph 13; chapter 4, paragraph 1.
In short, Lewis argues the only two alternatives besides accepting that Jesus is God is to view him as either an immoral liar or an insane person who did not realize he was lying. Most non-Christians don’t exactly like those two alternatives to this Jesus figure who often seems to them seems like a pretty solid dude–just not God. But Lewis will have none of this riding the fence garbage. A good moral teacher would not claim to be God without actually being so. To falsely claim such, he must needs be either a lunatic or a liar. Thus, as Lewis argues, this common tact of taking Jesus as non-God, non-lord, great-moral-teacher is off the table.