When I was a kid going to church, I got a prescription that was top down: Christ is god, Heaven is real, be good or else, believe what the books say. All other explanation was filling in the gaps below these claims. If Christ lived, what was his life like? Enter the Gospels. If God is all powerful, what exceptional work has he done? Enter the stories of the Old Testament. This was religion by deduction, with Christ being the unquestionable axiom. The literal truth of the story was all important, and the lessons less so. In my head: doubting whether Jesus lived was worse than disobeying the moral behaviors he prescribed.
At a certain point in my life, it all became too incredible. My understanding of the world was developing outside of the church and through an opposing methodology: bottom up. Fundamental yet simple ideas were built up to into reproducible, verifiable general claims. The apple falls from the tree at the same rate every time it drops. Enter Newton.
What's interesting about Mere Christianity is it tries to prove (the value or the fact or both), or perhaps tell the story of, Christianity by induction. It emphasizes the quality of the claims and of living those claims out, rather than make you feel simply guilty or dumb for not believing what supposedly should be obvious.
It's the first time a Christian work gave me the feeling that doing right was more important than belief in the supernatural, and that understanding what was right and wrong was more important than some vague emotional feeling of transcendence.
That being said, I'm still not convinced Christianity is the best articulation of morality or the best instruction set for behaving correctly, at least not for everyone. Rather, Christianity seems to be something of an intellectual fractal: at any scale of intelligence, it has a distinct compelling form or presentation.
For simple people, the mystical legend inspires and aligns; for these people, coming back from the dead and being all powerful are enough. For more curious people, there's a deeper explanation of perfection and humanity and intelligence vs. instinct, lessons that inspire just the same. Mere Christianity is an attempt at joining the two, emphasis on the latter. The distinction and its importance reminds me of the contrast between Sam and Liza Hamilton in East of Eden.
While C.S. Lewis doesn't concede that Christianity could just be an allegory — and in fact claims it can't be and that it must be actually true — he does allow for the possibility that his idea of Christ could be working in the hearts and on the minds of non-believers, perhaps slowly unveiling the truth (by the author's understanding) to them (i.e. the non believers). To me this is useful, because it means goodness and morality can be achieved without Christ, or as Lewis would argue, without the knowledge of Christ, at first. (Lewis argues this is what is going on in the BC religions and other "somewhat right" religions). Baby steps, if you will, but also a good argument for mutual compatibility and tolerance.
There's a good amount of silly stuff in here as well. Some of the comments on blacks, women, and gays are so silly and wrong. Granted, it's a book from the 1950s, and the other ideas are explanative enough to redeem the book without endorsing the wrong-headed ideas.
Specifically, on those wrong-headed ideas, take for example gay people. C.S. Lewis seems convinced that Christ and our conscience are constantly reminding gay people that their preferences are perverse, and women that they should be dependent, and blacks that they are inherently inferior. And subsequently, he's reminding them to not to stray. And maybe back in the day that was more true (that the internal reminder was there, not that these claims were actually true), a consequence of the guilt and social pressure and communal definition of these roles' place in the world. But in 2019, when so many gay people can and do live guiltless, diverse lives; and so many women are successfully independent, without sacrificing any of their loveliness; and so many blacks (not to mention biology itself) have demonstrated melanin's lack of influence on a person's humanity and intelligence and their right to be treated accordingly... These aren't propagandized claims that I think I believe in, but that my heart (and Christ?) tell me are actually wrong. These are truths I'd have to do work in order to disprove to myself. There's just too much evidence for these people's authentic claim at love in their own way, capacity regardless of gender, humanity regardless of race. It's ironic that Lewis, creator of so many fantastic worlds, lacks imagination in these cases — but it was a different time. Oppress a people for long enough and they will look like the dumb caricature in your head.
Then again, couldn't we, the contemporary world, take Lewis's own argument of Christ showing us what's right whether we know him or not, and use that argument to transitively justify our own intuitive and learned knowledge, the understanding that gays and blacks and women deserve to be respected and empowered and loved justified by saying that the knowing itself is the result of Christ's guiding hand? It seems you could, except when you then try to incorporate the scripture's specificity into the thing, you loose that dynamism.
And so at the end of the day I'm forced to choose between the literal truth of the canon and the intuited truth of a moral conscience. I don't think Lewis's Mere Christianity resolves this choice as well as he thinks it does, but it does give an elegant explanation of what Christianity is, in technical detail.