From Wikipedia with minor alterations by me:
C. S. Lewis used this bit of reasoning to show the logical inconsistency of believing that Jesus was not God, but was a great moral teacher.
Jesus claimed to be God.
Therefore he must be one of the following:
Lunatic: Jesus was not God, but mistakenly believed that he was.
Liar: Jesus was not God and he knew it, but said so anyway.
Lord: Jesus was God.
If Jesus was not God then he was not great or moral.
In his book, Reasonable Faith: Christian Truth and Apologetics, William Lane Craig says that Lewis’ premise is false that there are no alternatives available.
The whole argument rests on the accuracy of the Biblical account of Jesus’ life and that he claimed to be God.
[Posted from Ames, Iowa]