Harper Lee uses the character of Atticus Finch to tell the reader that courage comes in many different forms.  Atticus confesses to Jem that "Mrs. Dubose was the bravest person [he] ever knew," but Atticus demonstrates his own courage through his determination to defend Tom Robinson, despite the censure of his friends and neighbors.

Atticus Finch stands as a sterling example of a man of principle who is willing to practice his moral values despite censure by his community.  Although he does not wish to handle the case of Tom Robinson, for instance, he accepts it and defends the man with all the expertise that he can bring to the case.  When he believes that his son has killed Tom Ewell, Atticus makes no excuses for him, nor does he ask any favor of the sheriff, Heck Tate.

Harper Lee uses the character of Atticus Finch to tell us that....even people who are both wise and prudent can over-estimate the moral fiber of others. Atticus Finch's optimism regarding the eventual cooling of Bob Ewell's bitter anger nearly leads to the death of Jem and Scout. Though Atticus is right to presume that most people are really good at heart, once you meet them, he is wrong to assume that this is true of everyone. 

This is one of the most significat complexities offered in the novel. 

Harper Lee uses the character of Atticus Finch to tell us that there are good people in the world even when there are bad ones.  Atticus is one of the few people in the town willing to defend Tom Robinson, but there are others who are also on his side.