Lewis famously said that "all my seven Narnian books...began with pictures in my head. At first they were not a story, just pictures." Thus The Lion, the Witch, and the Wardrobe "began with a picture of Faun carrying an umbrella and parcels in a snowy wood." Just as we are recovering from the shock of that revelation, Lewis adds, "This picture had been in my mind since I was about sixteen. Then one day, when I was about forty, I said to myself: 'Let's try to make a story about it.'"
Just in case we might think that we cannot possibly have heard things correctly, Lewis also gave us another passage of similar import -- only more shocking. In countering the assumption of some of his readers that he "began by asking myself how I could say something about Christianity to children," Lewis claimed that "at first there wasn't even anything Christian about [the stories]."
The order of composition suggests an order of reading. If we follow the lead of Lewis himself, a major lesson we can learn from the Narnian stories is that they are first of all stories -- adventure stories, fantasy stories, children's stories. These narrative features are not simply "a disguise for something more 'adult'."
There is an image posted at the article that details the Biblical themes of the stories - basically they are a walk through the Biblical narrative: