The quote most associated with Harper Lee's masterpiece and the source of its title captures the overall theme of the novel most appropriately. Atticus presents his children with BB guns for Christmas one year, and leaves them with these instructions:
Shoot all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird.
In the novel, the mockingbird becomes a metaphor or symbol of Tom Robinson and Arthur "Boo" Radley. Robinson is falsely accused and then found guilty of a crime he didn't commit, while Arthur Radley has been kept under virtual house arrest since getting involved in some fairly harmless teenage mischief years before. Robinson ends up dead after he tries to escape his imprisonment, bringing his personal tragedy to a violent end; however, Radley escapes being made a local celebrity after he saves the Finch children from Bob Ewell, just assuring that this "mockingbird" is not shot.
I think aniother Quote is the final lesson of the book. Scout says, "Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough." I think this summarizes the majority of the story and that the lesson being taught throughout the book.