C.S. Lewis, one of the 20th century's top intellectuals, considered himself too smart for Christianity.

So how, at age 32, did he suddenly become one of its greatest advocates?

He was struck by a strange feeling — and something Tolkien said to him late at night…

C.S. Lewis's conversion didn't begin suddenly. He first began to feel a deep longing, pointing him to seek out the most beautiful things in life: music, art, romance. And yet, nothing he could find completely satisfied it.

He called this profound longing "joy", and intuited:

If we find ourselves with a desire that nothing in this world can satisfy, the most probable explanation is that we were made for another world.

This joy, he decided, must only be fulfilled by God. When we run from God, we become addicted to joy's temporary fixes (e.g. sex or substances). Still, he couldn't accept that the Christian story held any weight.

Lewis was always captivated by Pagan myths and the underlying messages they conveyed. But he saw Christianity as merely another iteration of the "dying-and-rising gods" story found throughout history. If he saw sacrifice or resurrection in Pagan and Norse myths (like Balder) he was drawn to it — finding it "suggestive of meanings beyond my grasp". But if he encountered the same themes in the Gospels, he admitted, then he resisted.

That's where a fateful conversation with his friend J.R.R. Tolkien (a devout Catholic) comes in. On a trip along Addison's walk in Oxford in 1931, Tolkien planted in Lewis's mind an idea…

While discussing myths, Lewis admitted however useful he found them, they never actually happened. Ultimately, they were lies.

"No," said Tolkien. "They are not lies." They in fact contain fragments of an eternal truth.”

Tolkien conceded that Christianity is indeed a myth, in the sense that it works on our imagination, and that it reflects the common patterns among myths. But Christianity, Tolkien said, is uniquely different — it's a true myth.

If the stories within ancient myths keep repeating themselves, doesn't that mean they're all pointing to something? What if the Christian story is the culmination of these myths, finally fulfilled by a real, historical event?

Continuing long into the night, Tolkien convinced Lewis that the old myths he loved were similar because they all pointed at the same truth. Those archetypal stories are like scattered fragments of a greater story — one that really happened.

A few days later, Lewis recounted this realization in a letter:

The Pagan stories are God expressing Himself through the minds of poets, using such images as He found there, while Christianity is God expressing Himself through what we call 'real things'. Therefore it is true, not in the sense of being a 'description' of God (that no finite mind could take in) but in the sense of being the way in which God chooses to (or can) appear to our faculties.

Once converted, Lewis felt the way Christianity had been taught to him in his youth was bland and prosaic. So, he wrote stories to reveal Christianity's engaging elements via old myths: like retelling the archetypal love story of Cupid and Psyche with a new message.

And moving on from myth, he wrote new stories as direct allegories for the Christian one — like Narnia.

Through his love of such stories, Tolkien helped Lewis solve his "joy" conundrum. Just like his own longing, the old myths reflected a kind of collective longing of men throughout history. These could never fulfil him since they weren't true, and authored by people.

As much as Lewis found meaning in old books and tales, the ultimate truth "was not in them, it only came through them." But if such a story were to really become true, it would solve everything...