When young fantasy readers first learn that the author of The Lord of the Rings once bro-ed down with the author of The Chronicles of Narnia, it’s as exciting as seeing two former enemies team up to fight the bad guys in our favorite adventure movie. And indeed, the relationship between J.R.R. Tolkien and C.S. Lewis, who first met on this day in 1926, is going to be the subject of an $18 million film helmed by the director of Con Air.

While not exactly the fodder for an action movie, their friendship was as fraught and productive as the quests they described in their influential works — although their battles were more theological and aesthetic than orcs vs. men.

Tolkien and Lewis were companions at Oxford, where they were both members of a group called the Coalbiters. That group evolved into the now famous Inklings, an informal association of literary pals who discussed their craft and workshopped each other’s writing, whether it was theology or the aforementioned elvish verses.

The devout Catholic Tolkien legendarily brought the doubting Lewis back into the fold of Christian faith, which put the latter’s creativity into overdrive. But then Lewis embraced Anglicanism so wholeheartedly that Tolkien ended up thinking his old friend was a little bit biased, and it stung when Lewis used terms like “papists.” Their greatest aesthetic arguments were about the use of Christian allegory in their work. While Tolkien purposefully submerged his Catholic themes deep within his very extensive narrative, Lewis did the opposite. Lewis’ great Narnian lion Aslan is a clear stand-in for Jesus, resurrection and all. And in The Last Battle, Lewis’ concluding tome, we encounter a harsh takedown of other religions and a version of the afterlife as a gated community which excludes lipstick-wearing Susan from its sanctuary.

Tolkien, whose fantasy world was rigorously supported and scaffolded by a system of languages and myths that he invented, was rather unimpressed by both Lewis’ overt Christian references and his mixing of Greek, Norse, medieval, and other kinds of mythology together — alongside appearances from contemporary figures like Santa Claus. On the other hand, the prolific Lewis pushed the more slow-moving Tolkien to keep going with The Lord of the Rings, which was finally published after years of work, and eventually found an unlikely fandom in the hippie subculture of the 1960s.

Although they drifted apart in later years, the authors still had an impact on each other. When The Fellowship of the Ring was completed, Lewis wrote:

No imaginary world has been projected which is at once so multifarious and so true to its own inner laws; none so seemingly objective, so disinfected from the taint of an author’s merely individual psychology; none so relevant to the actual human situation yet so free from allegory.

For his part, when Lewis died, Tolkien said, “this feels like an axe-blow near the roots.”

Beyond their arguments, their synergy launched an entire genre. In a Chronicle of Higher Education piece titled “Oxford’s Influential Inklings,” Philip and Carol Zaleski write that their coterie was something of a counterpart to the Bloomsbury group:

They shared much with Bloomsbury, including love of beauty, companionship, and conversation, but they differed from their older London counterpart in their religious ardor, their social conservatism, and their embrace of fantasy, myth, and (mostly) conventional literary techniques instead of those dazzling experiments with time, character, narrative, and language that mark the modernist aesthetic.

Reading Tolkien satisfies a longing for rich, exciting storytelling, full of suspense and major moral questions — these books resist the facile but remain uninterested in narrative tricks or showing of the authorial hand. The trilogy is an update of the kind of epic that Tolkien studied and translated, from Beowulf to Sir Gawain and the Green Knight, for the dilemmas of the 20th century. Together, he and Lewis essentially founded the fantasy genre, which, like any other, encompasses high heights (Game of Thrones, Harry Potter, etc.) as well as derivative trash. As the Zaleskis note,

Without the Inklings there would be no Dungeons & Dragons (and the whole universe of online fantasy role-playing it produced), no Harry Potter, no Philip Pullman (in his role as the anti-Lewis). Hollywood, the voice and arbiter of popular culture, has shifted dramatically toward mythopoeic tales; this is widely recognized to be the legacy of Tolkien, whose influence was disseminated by the 60s (“Frodo Lives!”) drug culture, itself a neo-Romantic movement that soon overflowed its banks.

The theological arguments in which the two engaged, combined with adventure stories they repurposed from mythology, have helped create a literary landscape where some of our most interesting ethical and philosophical questions are hashed out, in between chase scenes and armed confrontations.