I was 20 years old before I realized they were a Christian allegory and my very first thought was "But they're so Pagan!" Yes, I know in the very first book it talks about Sons of Adam and Daughters of Eve, but that never seemed much of a religious reference to me, (I remember I had to ask my parents what it meant) it was more just a way of clarifying which world you came from. Especially after reading Prince Caspian, where you first find out that the "humans" of Narnia are called Sons of Frank and Daughters of Anne.
I admit I was a little bummed when I first found out, but maybe because I had been reading them in a certain mindset for so long I got over it very quickly and viewed them the same way that I always have-as some of the best pagan fiction ever written! LOL Then I felt even better later when I found out that, although a very religious man, C.S. Lewis never intended for them to be taken as an allegory.