I hesitate to tackle the Chronicles of Narnia here, because I have such mixed feelings about the series. In some ways, I find them perfectly acceptable epic fantasy, with sweeping scope and some imaginative characters. Yet Lewis' prose can be drowsy (as can his friend Tolkien's in all honesty, a feature of their time and place and social status), and the narrative threads blurry. The Christian allegory is also a bit too overt for me, sort of like listening to a salesman tell me a funny story that I know ends with him asking me to buy his wares.
There are few classic scenes in children's fantasy literature as iconic as the Wardrobe, or characters as famous as Aslan. The Pevensie children are enjoyable protagonists, if a bit on the too-cute in an English pinafore sort of way, and their adventures are diverting. I found the heroic Peter and ingenious Lucy and inconstant Edmund to be a bit dull, and was interested more in older sister Susan, who on the face of it appears to be the most boring of the lot. She's gentle and kind, diligent with her magic bow, a bit of a reluctant adventurer, a little too ready to preach caution in her role as the timid mother figure of the quartet. But it is in Susan that Lewis weaves his most tragic (and controversial) subplot. She is the one who loses faith, who stumbles, and for her doubts she is refused entry into her father's house. There is no room for loss of faith in Aslan's Narnia or Lewis' theology.
It's become something of a feverish debate in literary circles, sufficiently worked-over to have earned the moniker "The Problem of Susan" (after Neil Gaiman's 2004 treatment of the issue). There are some who bristle at Lewis' ostracism of Susan, claiming that it seems to juxtapose with her discovery of sex (lipstick and nylons and invitations, in Lewis' neo-Victorian euphemism). Is the author suggesting that young women adhere to some standard of purity to retain their favored status? Or is Susan's transition to adulthood and her attendant doubts merely a temporary fall from grace? Lewis himself argued that her tale was an unfinished one. While she was not on the train that crashed and returned her siblings to Narnia, it is not clear that she could never return. Knowing some of Lewis' other work, I would be surprised if he did not believe that salvation is never permanently revoked, and that redemption is always possible.