C.S. Lewis might seem like his work would only be accessible to culturally Christian readers, but his treatment of religion itself in Mere Christianity should count for something in terms of philosophy and philosophical treatises. At the center of the analysis provided in the essays is the sense that somehow, community ought to be retained if at all possible.
In order to understand Lewis correctly, it's important to contextualize his writings in their historical domain. In the mid-20th century, Protestant Christianity was becoming so hyper-specific and niched that the common sense of unity and community that the church was designed to foster had splintered into various factions. The reader will find that Lewis is more interested in reunifying than drawing any divides, and that unity should extend to the original schism as well, reincorporating the Catholic tradition into the Protestant conception of the "Church proper." This is the main philosophical discussion at the heart of Mere Christianity, and the treatment of a divided church is explicitly mentioned as the reason for the book's writing, so no one has to wonder. Lewis's Mere Christianity is mainly an argument that no matter what denomination of Christianity one belonged to, the umbrella of "Christian" was broad enough to include them, and we should behave like a family, not as rival armies.
For those who don't share the Christian worldview, these things may not seem to have a place, but they absolutely do. Essentially, this is an academic defense of unity in spite of diversity. A non-Christian might have an experience with Lewis akin to a Christian's experience of Muslim scholars like Averroës. Although there is some religious disagreement, the philosophy translates, because it is rooted in the human experience.
Also, it's worth noting that Lewis makes arguments just like this throughout Mere Christianity. Ultimately, Lewis's writings must be a commentary on humanism and the way we view people who feel differently than us. He feels we should find anything and everything we have in common, so that our sense of community and family is not compromised, and although the arguments stay within the bounds of Christianity, these ideas are certainly important during the angry pluralism of the 21st century.
