It's taken from Mere Christianity, by CS Lewis (another TCC recommended read). It may be the most famous quote from the book and it speaks to the claims of Jesus Christ:
"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.' This 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 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 on His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to."
JLM