We can see that blacks as well as the white community accept racial segregation when Calpurnia takes Jem and Scout to her all-black First Purchase Church while Atticus is out of town. Lula, a black woman, asks Calpurnia about this:
“I wants to know why you bringin‘ white chillun to nigger church.”
“They’s my comp’ny,” said Calpurnia.
Lula is hostile to the children, while the rest of the congregation is welcoming: nevertheless, it is clearly very odd to them that whites are worshiping with blacks. Scout learns that the black church, being so poor, conducts services differently than her own church, but she gains knowledge that very few whites will ever possess.
The strict racial segregation of Maycomb is again a topic when Scout and Jem discuss Dolphus Raymond's mixed race children. When Scout asks Jem what "mixed race" means, he responds as follows:
“Half white, half colored. You’ve seen ‘em, Scout. You know that red-kinky-headed one that delivers for the drugstore. He’s half white. They’re real sad.”
“Sad, how come?”
“They don’t belong anywhere. Colored folks won’t have ‘em because they’re half white; white folks won’t have ’em cause they’re colored, so they’re just in-betweens, don’t belong anywhere.
Jem's comments show that he has internalized the strict color lines that keep black and white society separated in the South. A mixed race child has no place because there is no place for racial mixing in Maycomb. Jem does comment that Dolphus has sent two of his children north, where racial mixing is tolerated. This emphasizes the difference between the two regions of the country.
In discussing the upcoming Robinson trial with Jack during Christmas, Atticus alludes to the racism in Maycomb and the strict racial line he is stepping over by defending Robinson properly when he says,
You know what’s going to happen as well as I do, Jack, and I hope and pray I can get Jem and Scout through it without bitterness, and most of all, without catching Maycomb’s usual disease.
"Maycomb's usual disease" is racism, particularly the hysteria over the crossing of racial lines that Tom's alleged rape of Mayella represents.