Chapter 12 begins the second part of the novel and is the transition from childhood innocence and the harsh reality of life in Maycomb. Calpurnia is an important character in this chapter because she lives in both the world of the whites and the blacks. For the first time, they realize that Calpurnia has a whole other life apart from her life with the Finches.
That Calpurnia led a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one...
While Atticus is away at the state legislature, the children attend church with Calpurnia. The children are welcomed as honored guests at the First Purchase African M.E. Church in a way that Calpurnia would never be welcome at the "white" church.
On the way home, the conversation between the children and Calpurnia foreshadows the racial and class divisions that build as the story progresses.
For example, Calpurnia tries to explain to Scout and Jem why she talks differently when she is with other black people. She tells the children that even though she knows the difference between speaking properly and talking like "the rest of the colored folks," she would not fit in with her community if she spoke "white-folks talk."
The appearance of Aunt Alexandra at the Finch home at the end of the chapter symbolizes the encroachment of Maycomb's racism into the children's lives.