There is one way in which the movie did dilute the book's Christianity - to the best of my recollection there is no mention of Aslan being the son of the Emperor-over-the-Sea, no mention of the Emperor at all.
There isn't a lot in the book - Mr. Beaver so names Aslan when explaining him to the children, and Aslan says, "Work against the Emperor's magic?" in such a way that no one ever suggests such a thing again. In the movie I think he says "Work against the Deep Magic?"
An extremely significant omission.
Mrs VS
I think that given the way the death of Aslan closely paralleled the story of the Passion, it is not a significant omission to exclude one detail which doesn't come into play in the story and would only cloud the story further.
I am not a Christian, but I enjoyed the movie for what it was and am happy that Christian parents will have this splendid tool for their children to learn the story of Christ in a different form. I don't think they were trying to white out God or anything but not mentioning the Emperor, I just think it's not relevant to the story and certainly not needed to get the point across. I mean, come on, Susan and Lucy hugging Aslan in the garden before he makes his sacrifice and then going to collect his body after he's killed? You can't accuse them of cutting corners on the allegory.
There IS a mention of Aslan as the son of the Emperor-over-Sea. Don't remember exactly where but it occurs fairly early on . . . maybe in the conversation with Mr. & Mrs. Beaver?