Lewis credits Tolkien and their friendship as a primary reason for his Christian conversion. In 1929, they began to meet about Middle Earth and writing in general. Lewis converted to Christianity in 1931. That night in 1931 was a long one, lasting until four in the morning. Lewis discussed matters of faith with Tolkien and Dyson, a friend whom he had met a year earlier.
A few weeks later, Lewis explains more to Greeves. He says that his hold-up centered around redemption, and, "in what sense the life and death of Christ 'saved' or 'opened salvation to' the world." He couldn't understand how the death of someone else 2000 years ago could matter and help save a person -- him -- now. "Now what Dyson and Tolkien showed me was this," Lewis says,
that if I met the idea of sacrifice in a Pagan story I didn't mind it at all: again, that if I met the idea of a god sacrificing himself to himself I liked it very much and was mysteriously moved by it: again that the idea of the dying and reviving god similarly moved me provided I met it anywhere except the Gospels. The reason was that in Pagan stories I was prepared to feel the myth as profound and suggestive of meanings beyond my grasp even tho' I could not say in cold prose 'what it meant'.Lewis continues:
Now the story of Christ is simply a true myth: a myth working on us in the same way as the others, but with this tremendous difference that it really happened: and one must be content to accept it in the same way, remembering that it is God's myth where the others are men's myths. (October 18, 1931)The conversation with Tolkien and Dyson changed him. He became certain that Christianity fulfilled myth and was real, that it is the way in which God has revealed himself to humanity.
Ten years later, he referenced the occasion of coming to faith to Bede Griffiths, a student who came to Magdalen College in 1925, Lewis first year as a Fellow in English. Lewis wrote to him frequently. in a letter dated Dec. 21, 1941, he writes to Griffiths about the Inklings, a collection of friends including Dyson and Tolkien. "We meet on Friday evenings in my rooms," Lewis says, "theoretically to talk about literature, but in fact nearly always to talk about something better. What I owe to them is incalculable. Dyson and Tolkien were immediate human causes of my own conversion. Is any pleasure on earth as great as a circle of Christian friends by a good fire?"
I think Tolkien would agree with the image of friends around a fire, especially as we reflect on the many times we see the dwarves and Bilbo and the Fellowship around a fire, talking about adventures, legends and hopeful returns. Perhaps the Inklings proved fodder for such imaginative moments.
HaperOne has three volumes of Lewis's letters. They are a phenomenal resource.