In the early hours of 21 September 85 years ago, C.S. Lewis and J.R.R. Tolkien were debating the coherence of Christianity in one of the foundational moments that led Lewis to become a Christian.

Lewis, an Oxford professor and author, went on to become one of the great bastions of 20th century Christianity against a growing tide of atheist philosophy. As well as the Narnia series he went on to write and lecture in defence of faith. His most famous apologetic, Mere Christianity, is still widely read today.

On the anniversary of this significant date, here are 10 quotes from Lewis on the case for Christianity.

1. "Christianity, if false, is of no importance, and, if true, of infinite importance. The one thing it cannot be is moderately important."

2. "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."

3. "To be ignorant and simple now – not to be able to meet the enemies on their own ground – would be to throw down our weapons and to betray our uneducated brethren who have, under God, no defense but us against the intellectual attacks of the heathen. Good philosophy must exist, if for no other reason, because bad philosophy needs to be answered."

4. "Supposing there was no intelligence behind the universe, no creative mind. In that case, nobody designed my brain for the purpose of thinking. It is merely that when the atoms inside my skull happen, for physical or chemical reasons, to arrange themselves in a certain way, this gives me, as a by-product, the sensation I call thought. But, if so, how can I trust my own thinking to be true? It's like upsetting a milk jug and hoping that the way it splashes itself will give you a map of London. But if I can't trust my own thinking, of course I can't trust the arguments leading to Atheism, and therefore have no reason to be an Atheist, or anything else. Unless I believe in God, I cannot believe in thought: so I can never use thought to disbelieve in God."

5. "If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world."

6. "All I am in private life is a literary critic and historian, that's my job... And I'm prepared to say on that basis if anyone thinks the Gospels are either legends or novels, then that person is simply showing his incompetence as a literary critic. I've read a great many novels and I know a fair amount about the legends that grew up among early people, and I know perfectly well the Gospels are not that kind of stuff."

7. "If Christianity was something we were making up, of course we could make it easier. But it is not. We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with Fact. Of course anyone can be simple if he has no facts to bother about."

8. "If you had gone to Buddha and asked him: 'Are you the son of Brahma?' he would have said, 'My son, you are still in the vale of illusion.' If you had gone to Socrates and asked, 'Are you Zeus?' he would have laughed at you. If you had gone to Mohammed and asked, 'Are you Allah?' he would first have rent his clothes and then cut your head off. If you had asked Confucius, 'Are you Heaven?' I think he would have probably replied, 'Remarks which are not in accordance with nature are in bad taste.' The idea of a great moral teacher saying what Christ said is out of the question. In my opinion, the only person who can say that sort of thing is either God or a complete lunatic..."

9. "If you are concerned about your loved ones that won't be in Heaven, the most irrational thing you could do if you are truly concerned about those on the outside is to remain outside yourself."

10. "I believe in Christianity as I believe that the sun has risen, not only because I see it, but because by it I see everything else."