What is the moral lesson of the story?

The Magician's Nephew contains allegories for many tenets of Christianity; naturally, then one of the primary themes of the book is faith. One strong example of faith in the novel is when Aslan sends Digory to pluck just one fruit from a special tree for the purpose of protecting Narnia. Digory desperately wants to bring fruit to heal his mother, but he knows this would violate the inscription on the garden; despite the Queen's tempting, and Digory's own longing, he has faith in Aslan's guidance, and he lets this faith be his guide. Ultimately he is rewarded with fruit from the newly-planted tree of life in Narnia, and his mother recovers. The theme of faith recurs both in this novel and in all of the Narnia chronicles.