Writing of Jesus, Lewis said,
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 a 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 moral teacher. He has not left that open to us. He did not intend to."Thus Lewis set up what has become known as the Lunatic, Liar, Lord trichotomy (or the Mad, Bad, or God) trichotomy.
Before my mother rues the day she bore me, I assure you that I agree with Lewis in context most wholeheartedly that in the light of the gospels, the option of 'Jesus was a great moral teacher' is patronising nonsense. We either recognise the Son, continue in rebellion and kill him that we might play at ruling, or we fall at his feet and call Him Lord and God.
However, there are two issues with the common usage of the trichotomy which I will address. If I'm right then hopefully this will help us witness better. If not, let me know.
Firstly, some say that this is a false trichotomy - that these things are not categorically mutually exclusive and exclusive of other options. For instance, there are those who say that someone delusional about self-deity may be functionally sane in other areas of life. Even if one could not grasp this or another possibility for the options at the moment, it must be admitted that the options are not as closed as we would like them to be. However, this does not mean that we cannot be certain nor that we shouldn't use our present understanding; it does mean that setting up a trichotomy and discounting 2 of the choices is not a water-tight means of arriving at certainty. So while I don't say that Lewis' trichotomy is necessarily fallacious, we should note that it is not an ultimate measure and means to certainty. It's a limited tool for the limited comprehending of his revelation.
Secondly, and more importantly, the trichotomy rests on certain presuppositions. This is always the case with a logical argument. Lewis was not ignorant of this (this quote has a larger context!), but it seems that most Christians using his trichotomy ignore the necessary presuppositions, treating it as if it had none. A proof does nothing if the person with whom you're reasoning doesn't accept your presuppositions. In this case, it only holds on the presuppositions that the gospels are faithful accounts of historical events (and experiencially that the person with whom you are discussing is familiar with Jesus' life and claims from them - but we'll assume that because it can always be remedied). Taking it a step further back, it relies on presuppositions that there are such things as historical events and that there can be faithful accounts despite a lack of objectivity! If the trichotomy is used as if it's a water-tight argument without any mention of those presuppositions or any accord on those presuppositions, then it's not got a leg to stand on. So the person may accept the trichotomy as a useful bit of reasoning, but not accept that the gospel writers accurately recorded Jesus' life and claims.
With both these problems, we rely on the testimony of the gospel writers. Relying on testimony is not blind or irrational: we all rely on testimony a dozen times a day in what we do. But the issue of testimony is essentially something for another post. My point in referring to it for now is that the common usage of the Lunatic, Liar, Lord trichotomy, is forced, because it doesn't acknowledge the necessity of its presuppositions or ultimately of faith. It is not self-contained proof. We must recognise that it depends on belief in the testimony of the gospels and use it appropriately.
In other words and more broadly speaking, we can't philosophically reason our way to God irrespective of faith in revelation (through testimony). If we could, our own reason would be effectively our god, and revelation redundant. However, recognising its limits, reason may be used as an aide - just not relied upon. Revelation may be reasonable in a sense, but reason is not revelation.
All that for the sake of a carelessly unqualified aside - I must be more careful!