Atticus is a principled man, and so he takes great care to ensure that Scout and Jem both grow up to live as moral adults. He gives them a great deal of advice throughout the novel, both directly and indirectly. One of his most important lessons is his instruction to be the same person in all areas of life, whether one is at home or in public.
This idea is fundamental to the novel. Since he cannot adopt a fake persona, Atticus must live up to his true nature and honestly defend Tom Robinson, though he knows that the public expects him to live up to their racist expectations and allow Robinson to be charged guilty of a crime he didn't commit. Atticus faces a great deal of criticism, and even violent threats, as a result of his actions. However, by being the same person in public as he is at home, Atticus teaches Scout and Jem the importance of living authentically and striving to think for oneself, no matter what public opinion might be. By teaching them this lesson, Atticus takes an important step in ensuring his children grow up to be principled and honorable adults.