What reasons does Atticus give for why its a sin to kill a mockingbird?

Atticus says that killing a mockingbird is a sin as they do nothing against us, all they do is sing to try and please us.A mockingbird symbolizes as inicent, And "to kill someone's inicence is a sin" is what Atticus is trying to refer to. Also this is when he uses The Bible to explainsomething to Scout and Jem.
It was considered a sin To Kill a Mockingbird. It was said that all they do is sing their hearts out without bothering anyone or eating their crops.