By Justin Taylor, The Gospel Coalition.
C. S. Lewis popularized the argument that Jesus was either a liar or a lunatic or the Lord. But, as Kyle Barton has shown, he didn’t invent it.
In the mid-nineteenth century the Scottish Christian preacher “Rabbi” John Duncan (1796-1870) formulated what he called a “trilemma.” In Colloquia Peripatetica (p. 109) we see Duncan’s argument from 1859-1860, with my numbering added:
Christ either  deceived mankind by conscious fraud, or  He was Himself deluded and self-deceived, or  He was Divine. There is no getting out of this trilemma. It is inexorable.
In 1936, Watchman Nee made a similar argument in his book, Normal Christian Faith. A person who claims to be God must belong to one of three categories:
First, If he claims to be God and yet fact is not, he has to be a madman or a lunatic.
Second, if he is neither God nor a lunatic, he has to be a liar, deceiving others by his lie.
Third, if he is neither of these, he must be God.
You can only choose one of the three possibilities.
If you do not believe that he is God, you have to consider him a madman.
If you cannot take him for either of the two, you have to take him for a liar.
There is no need for us to prove if Jesus of Nazareth is God or not. All we have to do is find out if He is a lunatic or a liar. If He is neither, He must be the Son of God.