In my youth, my parents read us the full Narnia series, and being the smart children we are, we liked them. Growing older I became more and more aware of the Christian symbolism through out (especially in The Lion, The Witch, and the Wardrobe), but I became even more aware of the anger at the symbolism that people seemed to have developed over time.
The general source of this anger seems to be the fact that the Christian symbolism is so blatant. I actually know people who refuse to read the series, because of the symbolism. Imagine choosing not to read something because of the Christian symbolism. Sure, the character Aslan screams "I'm an image relating to Jesus Christ", but it does not stop the story from being interesting. Nor does it stop Lewis from using pagan symbols as well, as an online article from www.marxmail.org that Columbia University seemed to find interesting enough to re-post:
"If it is meant as Christian propaganda, one has to wonder if it is subverting its own goals through the inclusion of witches, fauns, centaurs and other creatures drawn from the ranks of mythology. Furthermore, the return of Christmas in this tale seems closer to the pagan roots of this holiday than to celebrating Jesus' birth. After all, putting up a pine tree as a symbol of the oncoming spring would owe more to Nordic ritual than the sort of austere Anglican theology favored by Lewis."
The author, who I had issues locating the name of, has a valid point: Lewis does use pagan imagery along side of Christian imagery. This mimics most of the religious growth we see. Christianity and Paganism basically have grown up together, essentially teaching other and each taking bits and pieces of the other. Does knowing this help calm the outrage of the over board Christian themes?
Besides that, if we stop reading pieces just because of blatant Christian themes, think of how many authors that cuts from our reading pool: Tolkien, Chesterton, Koontz... Heck, even Rowling admits to Christian themes in the Harry Potter series. The fact of the matter is that Christianity is a large part of our culture and had been for quite some time. Even those who are not Christian know the themes and references, making them universal. While it seems like the references are slowing down in our culture, it still makes sense to use them and to try and avoid them? Well, that's just impossible.
Links on the Symbolism