Lewis and fellow novelist J. R. R. Tolkien were close friends. Both authors served on the English faculty at Oxford University, and both were active in the informal Oxford literary group known as the "Inklings".
Lewis quoted Lucretius (De rerum natura, 5.198–9) as having one of the strongest arguments for atheism:
Nequaquam nobis divinitus esse paratam
Naturam rerum; tanta stat praedita culpa
In a much-cited passage from Mere Christianity, Lewis challenged the view that Jesus, although a great moral teacher, was not God. He argued that Jesus made several implicit claims to divinity, which would logically exclude this:Had God designed the world, it would not be
A world so frail and faulty as we see.
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.
This argument, which Lewis did not invent but developed and popularized, is sometimes referred to as "Lewis's trilemma". It has been used by the Christian apologist Josh McDowell in his book More Than a Carpenter (McDowell 2001). Although widely repeated in Christian apologetic literature, it has been largely ignored by professional theologians and biblical scholars.
Lewis's Christian apologetics, and this argument in particular, have been criticized. Philosopher John Beversluis described Lewis's arguments as "textually careless and theologically unreliable," and this particular argument as logically unsound and an example of false dilemma. Theologian John Hick argues that New Testament scholars do not now support the view that Jesus claimed to be God, New Testament scholar N. T. Wright criticizes Lewis for failing to recognize the significance of Jesus' Jewish identity and setting – an oversight which "at best, drastically short-circuits the argument" and which lays Lewis open to criticism that his argument "doesn't work as history, and it backfires dangerously when historical critics question his reading of the gospels," although he believes this "doesn't undermine the eventual claim."
Lewis used a similar argument in The Lion, the Witch and the Wardrobe, when Digory Kirke advises the young heroes that their sister's claims of a magical world must logically be taken as either lies, madness, or truth.
One of the main theses in Lewis's apologia is that there is a common morality known throughout humanity. In the first five chapters of Mere Christianity Lewis discusses the idea that people have a standard of behaviour to which they expect people to adhere. This standard has been called Universal Morality or Natural Law. Lewis claims that people all over the earth know what this law is and when they break it. He goes on to claim that there must be someone or something behind such a universal set of principles.
These then are the two points that I wanted to make. First, that human beings, all over the earth, have this curious idea that they ought to behave in a certain way, and cannot really get rid of it. Secondly, that they do not in fact behave in that way. They know the Law of Nature; they break it. These two facts are the foundation of all clear thinking about ourselves and the universe we live in.Lewis also portrays Universal Morality in his works of fiction. In The Chronicles of Narnia he describes Universal Morality as the "deep magic" which everyone knew.
I have met people who exaggerate the differences, because they have not distinguished between differences of morality and differences of belief about facts. For example, one man said to me, "Three hundred years ago people in England were putting witches to death. Was that what you call the Rule of Human Nature or Right Conduct?" But surely the reason we do not execute witches is that we do not believe there are such things. If we did – if we really thought that there were people going about who had sold themselves to the devil and received supernatural powers from him in return and were using these powers to kill their neighbours or drive them mad or bring bad weather, surely we would all agree that if anyone deserved the death penalty, then these filthy quislings did. There is no difference of moral principle here: the difference is simply about matter of fact. It may be a great advance in knowledge not to believe in witches: there is no moral advance in not executing them when you do not think they are there. You would not call a man humane for ceasing to set mousetraps if he did so because he believed there were no mice in the house.Lewis also had fairly progressive views on the topic of "animal morality", in particular the suffering of animals, as is evidenced by several of his essays: most notably, On Vivisection and "On the Pains of Animals."
Several C. S. Lewis Societies exist around the world, including one which was founded in Oxford in 1982 to discuss papers on the life and works of Lewis and the other Inklings, and generally appreciate all things Lewisian. His name is also used by a variety of Christian organisations, often with a concern for maintaining conservative Christian values in education or literary studies.