In To Kill a Mockingbird, Harper Lee emphasizes as a major theme of the story the racism endemic to the society in which she was raised and in which her young protagonist, Scout, is herself growing up. The most important conflict in Lee's novel is associated with that racism, exemplified in the approaching trial of Tom Robinson, a desperately poor African American man accused of raping a white woman--as inflammatory an accusation as could be made in that place and time. Scout's father, Atticus, the moral compass of the novel, has agreed to defend Tom despite the tension surrounding the case and the problems he knows it can bring down upon his family.
In Chapter 12, we see the mistrust and suspicion with which some members of the black community now regard any white visitors to their church when Lula confronts Calpurnia and questions the right of white children to visit a black church. Capurnia prevails in the encounter and the congregation welcomes Scout and Jem.
As Calpurnia and the children return to the Finch home, Scout inquires of the housekeeper the reason for the reverend's plea during the church service for financial help for Tom Robinson's wife and children. Calpurnia's reply shifts the focus of Maycomb's racism back to the majority white population:
"On the way home I asked Cal why no one would hire Helen. She told us that it’s because of what folks say Tom’s done. I didn’t know what Tom had done so I asked. Cal sighted, 'old Mr. Bob Ewell accused him of rapin’ his girl an’ had him arrested an’ put in jail—'"
Again, we see the depths of the racism prevalent in Maycomb, Alabama. Tom stands accused of raping a white woman, Mayella Ewell, eldest child of the virulently racist Bob Ewell. Any association, then, with Tom Robinson is poisonous, and Tom's wife, Helen, is considered guilty by association with her husband.
With respect to the final sentences of Chapter 12, one would be well-advised to go back and reread the opening sentences of this chapter. Scout's brother Jem is now twelve years old, and maturing emotionally as well as physically. In the chapter's opening passage, he angrily yells at his younger sister, “It’s time you started bein’ a girl and acting right!” This criticism by Jem presages the chapter's ending and the events to follow. The final passage is as follows:
"As we approached the house Jem told me to look on the porch. I looked and saw Aunt Alexandra sitting in a rocking chair."
These two sentences are important because, it will be made very clear, Aunt Alexandra has arrived with the intention of staying indefinitely for the express purpose of ensuring that Scout--Jean Louise, to Alexandra, in keeping with the latter's emphasis on the former's requisite femininity--is raised as a lady. Aunt Alexandra is critical of Scout's tomboyish ways and is determined to see that Atticus's shortcomings as a single parent are rectified by her, Alexandra's, authoritative and feminine presence.
Chapter 12 marks the children's visit to Calpurnia's church. Though most of the parishoners are more than welcoming to the Finch children, Lula, a "tall Negro woman," makes Scout and Jem feel out of place at the church. Lula asks Cal why she has brought white children to the AME Zion church, and says,
You ain't got no business bringin' white chillun here--they got their church, we got our'n. It is our church, ain't it, Miss Cal?
When Jem suggests that they go home because they're not wanted, Zeebo explains that Lula is "contentious because Reverend Sykes threatened to church her." It's clear that Lula is well aware of the racial climate and tensions in Maycomb, and she feels that the Finch children shouldn't be present at her church.
The last sentence of the chapter announces Aunt Alexandra's arrival, and in the beginning of Chapter 13, we learn that Alexandra has arrived to act as a "feminine influence" in the Finch household.