"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."
C.S. Lewis was drawing on the aut Deum aut malum ("either God or a bad man") formulation of earlier Christian evangelists, but his expanded version stuck. In its simplest form, the trichotomic choice he proposes runs as follows:
If Jesus claimed to be God, then:
1) He was God, and He knew it (Lord)
2) He was God, and He didn't know it (a logical impossibility)
3) He wasn't God, and He knew it (Liar)
4) He wasn't God, and He didn't know it (Lunatic)
In its conditional form, the statement is uncomplicated and unavoidable. The only question is whether or not Jesus actually claimed to be God. Thus we arrive at a modified trichotomy:
If "Jesus" means the historical person as depicted in the Bible, then Jesus claimed to be God.
If Jesus claimed to be God, etc. (see above).
Please note that this conditional does not state that the Bible accurately depicted the historical person Jesus ben-Joseph. The modified trilemma only states that the Jesus presented by the Bible did make claims to be divine.
There are internal textual evidences for this:
1) Jesus claimed to have the authority to forgive any sin (Luke 7:48) -- the sole province of God.
2) Jesus claimed to have the authority to judge the world (Matthew 25:31) -- again, that was God's thing.
3) Jesus responds "before Abraham was, I AM" to the Pharisees (John 8:58), identifying Himself by the Name of God. At this undeniable assertion of divinity, the Pharisees sought to stone him, and
4) Peter called Him the Christ, the Son of God, (Matthew 16:16) for which Jesus praised him, and stated that He would build His church on Peter, and by implication on Peter's confession.
There are also a number of external considerations:
5) If Christ didn't claim to be divine, how did His followers manage to become so convinced that He did? Every child in first century Judea could have recited by heart the Deuteronomical Shema ("Hear, O Israel, the Lord your God, the Lord is one"). Is there a people in the history of the world who would have been less likely to confuse a human with the One True God? Yet we're supposed to believe that Jesus managed to inadvertently convince His disciples that He (a mere human) was somehow divine, despite their repeated obtuseness (recorded by Scripture) and the conservative assumptions of their Jewish traditions. At the very least, if Christ didn't claim to be God, then he must have been an almost dangerously incompetent teacher.
6) The Gospel of John begins with the plain assertion that "In the beginning was the Word, and the Word was with God, and the Word was God... and the Word was made flesh." That is John's gospel begins with the statement that Jesus Christ is God incarnate. It's rather absurd to argue that John intentionally omits any Christological claim to divinity, when John clearly thinks that Jesus was God and that He had made such claims.
7) The same principle holds for the other authors of the Gospels. Those who deny that Jesus ever made such claims to be divine, accept that the so-called mistake originated from His disciples. I don't know a single person who questions whether or not Christ's earliest disciples were convinced of His divinity, at least in their public testimony. Considering, then, that the other authors of the Gospels were convinced of His divinity, it is simply nonsensical to suppose that they would have carefully omitted any record of such claims.
Again, the point isn't that the Gospel accounts are accurate. I think they are, but that it's irrelevant to this modified trilemma. The point is that you have to dispute or discount the Biblical account of Jesus' life in order to deny that Christ claimed to be the divine Son of God.
The final evidential point is that those Biblical accounts are the only widely accepted documentary evidence of Jesus' life and teachings remaining extant. Without the Gospels, our knowledge of the "historical Jesus" would be pretty meager indeed, and indeed I would argue that it's impossible to construct a meaningful account of Jesus ben-Joseph without relying on portions of the Gospels.
But the only way to manage this without including Christ's divinity claims is by importing some a priori hermeneutic standards to reject the distastefully orthodox content while retaining the passages that comport with a more vaguely universalist theology. But this "Jesus Seminar" approach to Biblical hermeneutics is so susceptible to preconceived biases, that it really doesn't bear much resemblance to honest textual exegesis or good faith argumentation.
The Christian trilemma is not the only set of options available. But in my opinion it is the only set of options that takes the Biblical accounts of Jesus' life seriously.