What is the key theme of To Kill a Mockingbird?

There are so many themes in this wonderful book. But I would argue that the key theme is loss of innocence. To Kill a Mockingbird is a coming-of-age story, detailing Scout's transition to young adulthood. In her blissful innocence, Scout is initially unaware of the numerous evils that exist in Maycomb, not the least of which is widespread racial prejudice. Yet, as the story progresses, Scout becomes gradually more aware of her surroundings, realizing in due course just how scary a place the world can be.
As with most children, Scout is forever asking questions about the things that don't make sense to her. Fortunately, Atticus is on hand to provide wisdom and loving guidance to his daughter, helping her to understand the complexities of life. Thanks to Atticus, Scout comes to realize that life isn't all black and white; there are numerous shades of gray. And although Maycomb may not be some kind of Shangri-La, it's not simply a hothouse of racism and social snobbery either. There are good people in Maycomb; Scout knows this from her own experience with the likes of Miss Maudie, Calpurnia, and, eventually, Boo Radley. And thanks to these good people, Scout's loss of innocence results not in disillusionment and cynicism, but rather in a much more complete understanding of human nature, which acknowledges the co-existence and complex interaction of both the good and the bad.