When I was 16, I stayed with friends of my parents while traveling. I saw a book analyzing Narnia, and it talked about CS Lewis as a Christian theologian. My response was: Wow, that's fascinating; I guess he managed to keep that out of the Narnia books, right?
But then I kept reading that book about CS Lewis, and it described how Narnia was all Christian, and how its symbolism, and characters, and language, were all about Christianity. I was absolutely shocked. How could I have missed it?
But I missed it because I was the wrong audience. The "dog whistles," as they're known in the political realm nowadays, failed to work for me, because I hadn't been raised to think in those terms.
By the way, I encouraged my kids to read Narnia I even read many of the books to them when they were younger. But I told them that the last book would be really weird, and that it reflects the author's Christian perspective. None of them got into the series as much as I did, so much less explaining had to happen -- but I did talk about it with them, and I think that they were almost as surprised as I was.