Atticus's respectful treatment of Calpurnia--indeed, he states that she is part of the family--is an indirect lesson to both children about tolerance. Atticus makes clear to Jem and Scout in no uncertain terms that they are to mind her; when Atticus is called out of town, Calpurnia stays with the children, and even takes them to church with her. This shows us that she is confident in her place in the Finch family, because she did not feel compelled to ask Atticus's permission, nor did she seem the least bit nervous about the prospect of taking the kids with her. Had Aunt Alexandra been in charge, that visit never would have happened.
Calpurnia scolds Scout about her commenting on Walter Cunningham's manners, early in the novel. The fact that Calpurnia, a black woman and employee of a white man, feels comfortable correcting the white daughter of her employer, shows us that Atticus holds her in the highest esteem.
Regarding the controversy that the Tom Robinson trial is sure to ensue, Atticus wryly comments:
"Why reasonable people go stark raving mad when anything involving a Negro comes up, is something I don't pretend to understand."
Regarding the trial, Atticus also gives the reader insight into his beliefs about people in general, when he observes:
"The one place where a man ought to get a square deal is in a courtroom, be he any color of the rainbow, but people have a way of carrying their resentments right into a jury box. As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it—whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash."