In Chapter 10 of To Kill a Mockingbird, explain the only sin Scout has ever heard Atticus mention.
Jem and Scout were ecstatic about receiving the gifts they had most wanted from Atticus for Christmas--air rifles--but their father refused to teach his children how to shoot them. So, Atticus's brother, Jack, was left with that responsibility, teaching Jem and Scout "the rudiments thereof." Their Uncle Jack told them that "Atticus wasn't interested in guns," but he didn't tell them about his brother's secret--one that Jem and Scout would discover at the end of the chapter. Atticus did caution Jem that
"I'd rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird."
Since that was the only time Scout had ever heard Atticus "say it was a sin to do something," she decided to confer with Miss Maudie about it. Maudie explained to Scout that mockingbirds were harmless creatures that only "make music for us to enjoy... That's why it's a sin to kill a mockingbird."