The book, which, along with the others in the series, I have read every year since I was 8 (and I will not admit how many times that is...), is about the rise of atheism. What the film lacks is a deliberate confrontation between faith and atheism. Sure, there were little hints, like when Aslan tells Lucy that just because her friends lack faith doesn't mean...blah blah. But that's a lecture, and who wants a lecture in an adventure film. In the book, that lecture is personal advice during a very confessional moment. You must do what you think is right, even if others tell you that you are wrong. Period.
In the book, Miraz champions atheism, and Dr. Cornelius, who is the Narnian equivalent of a priest-living-in-hiding, is the teacher of the faith who saves Caspian from a faithless world. Miraz tells Caspian that he has been told fairy tales, and Caspian despairs; Cornelius tells Caspian the tales are true, and Caspian rejoices. Caspian is forced on a pilgrimage and finds the Truth, and the Truth, ultimately, defeats the faithless. We see none of that in the film, and that's a shame. The rise of faith, and of the faithful, is the real story in Prince Caspian.