They were part of the literature discussion group The Inklings, which met mostly on Tuesdays at The Eagle and Child pub in Oxford.
Tolkien is the one that convinced Lewis to leave atheism behind and become a Christian. Tolkien's poem "Mythopoeia" - https://en.wikipedia.org/wiki/Mythopoeia_(poem) - was instrumental in that process.
It's worthwhile to read it, but as a warning, it was written from one (highly educated) friend to another (highly educated friend), and it is not easy to understand what Tolkien is trying to say about the nature of reality without reading through it a few times: http://home.agh.edu.pl/~evermind/jrrtolkien/mythopoeia.htm
I highly recommend delving more into both Tolkien and Lewis, if you haven't. Basically this single poetry line is reflected in so much of both their works: Silmarillion has several threads of this sentiment woven into it.
Tolkien is the one that convinced Lewis to leave atheism behind and become a Christian. Tolkien's poem "Mythopoeia" - https://en.wikipedia.org/wiki/Mythopoeia_(poem) - was instrumental in that process.
It's worthwhile to read it, but as a warning, it was written from one (highly educated) friend to another (highly educated friend), and it is not easy to understand what Tolkien is trying to say about the nature of reality without reading through it a few times: http://home.agh.edu.pl/~evermind/jrrtolkien/mythopoeia.htm