Atticus is the somewhat reluctant moral 'hero' of To Kill a Mockingbird. Before the novel begins (as it were), Atticus is part of a section of Maycomb society that might be described as liberal and non-racist. Others are such characters as Miss Maudie, Heck Tate, Doctor Reynolds, Dolphus Raymond and the people who come to the Finch household to talk with Atticus shortly before the trial. These are all 'good' people but they are fairly passive members of society and there is no evidence of any of them doing anything proactive to change the racist environment in which they live. However, Atticus is thrust into an active role, partly through Judge Taylor and Heck Tate pointing the finger at him and partly because his own conscience dictates it. He thus finds himself representing those in the community who know that things must change, slowly perhaps and in small steps, but inevitably.
Atticus Finch is the moral center of the novel for a number of reasons. The reason that stretches throughout the novel is that he tries to teach Jem and Scout what it means to be a good person. However, he could do that and just be preachy.
Three major things make him the moral center of the novel. First, his response to the rabid dog. Atticus steps up to do his duty when the dog threatens people, and at risk to himself. That's a symbolic lead up to the second thing, which is Atticus' defense of Tom. He tackles a complex political issue, he does the right thing, and he does it with class. Third, his influence on others. Look at Mr. Tate's dealings with Atticus in Chapter 30.
umm he teaches Scout and Jem to do what is right by teaching them life lessons and defending tom robinson. bcuz it was right