In my opinion, over-zealous Christian readers are pushing things in finding Christian symbolism in EVERYTHING in the Narnia books. To understand this, you have to understand CS Lewis. He LOVED mythology! He studied the Pagan stories, as well as just about anything else written down he could get his hands on. He found no fault in pre-Christian Pagans for being Pagan. It simply did not make sense to him to criticise people for not being Christian when the religion didn't exist yet. He also felt that Jesus was the fulfillmet of not only the Jewish prophecies, but of many other cultures. He believed the reason Jesus's life paralells so many Pagan stories is that God gave these cultures foreknowledge of what was to come. Now, I'm not saying I agree with all of this... just that it's what he believed.
So he DID give Narnia Pagan elements. Mr. Tumnus is one of those Pagan elements, and you'll notice he's a "good guy".
But CS Lewis was a Christian, and he was very strong in his faith after he found it again. His mother died when he was a child, and he spent years being an athiest out of anger toward God. His logic was, "You took my mother, so I'm going to get back at you by not believing in you!" In my opinion, this is not TRUE athiesm. You have to believe in someone to be angry enough to want to get back at them. But it was the course he took for many years. When he turned back to God, he was overjoyed to find some semblance of peace and fulfillment in his life, and it colored everything he wrote for the rest of his life.
CS Lewis once said about Aslan that he is not supposed to directly represent Jesus, but he is supposed to be a Christ-like character. His thinking was, "What if there was another world, different from ours, that also needed salvation? How would the story be similar? How would it be different?" This is why the story of Aslan's death and resurrection carries elements of the story of Jesus, but if you try to follow along in the scripture, there are many differences. Because it's not supposed to be the same story. Lewis wanted the story to draw people toward Jesus, but he didn't want to simply re-write what was already written in the bible. And he also wanted the story to be appreciated and enjoyed as a good story... not JUST a religious teaching tool.
If anything, I like to think of Narnia as a place where Paganism and Christianity can peacefully co-exist. It's not a peaceful place, not all the time... but they have bigger things to worry about than who's worshipping who, and they all come together for the good of Narnia. It would help our world immensely if we could do the same.