C. S. Lewis popularized the argument in his writings. Was Jesus a lunatic, a liar or the Lord? More importantly, is the question still valid? Is the trilemma still a valid argument for the deity of Jesus Christ?
First we must remember why the trilemma was originally posed. Lewis is attempting to refute the designation of "good moral teacher" which is often attributed to Jesus. He states that there is no logical room for that choice; instead restricting the possibilities to three (four, if you count "legend", but no one seems to deny the existance of a man named Jesus anymore). Simply put, Jesus was right, He was wrong or He was nuts. The other important foundational consideration is the veracity of the Gospel accounts of Jesus' words. If one doesn't accept the accuracy of Scripture, the trilemma falls flat. However if one considers the Gospels as true, or at least reasonably close to true, one must then deal with the deity claims of Jesus.
Lewis' choices are simple. First, was Jesus a lunatic? Here he pulls out the most memorable illustration I've read in Christian literature. As a lunatic, Jesus would be on the same level as a man who claimed to be a poached egg. Now there's a mental picture for you! And certainly a lunatic could claim to be God, but I'd rather not follow a lunatic. And the rest of Jesus' reported behavior doesn't indicate someone with leanings toward breakfast food. Lunatic doesn't appear to be a logical choice.
Was Jesus a liar? Lewis points out that a liar wouldn't be a proper characteristic of a good moral teacher. Years ago, that statement may have been easier to accept. These days I have heard people claim that there are good moral teachers all over the place who are far from perfect. In fact, if we are to accept moral teachings from anyone then we are learning from sinners -- often liars. There is no one perfect, right? So the new argument is that Jesus could be both a liar and a good moral teacher.
However, the modern-day examples of moral teachers are mostly without any claims to deity. That puts Jesus up on another level in my book. Claiming to be God is not the same as adultery or plagerism or cheating on income taxes. I would assert that one cannot falsely claim to be the ultimate moral authority -- the way, the truth and the life -- and still be a good moral teacher. Yet the argument is weakened by those who see no difference.
Is the trilemma still a valid argument? I would say yes, but it is not as effective today with people who will accept moral teaching from immoral sources.