When Calpurnia takes Jem and Scout to her church, the children learn that there are similarities (the sermon) and differences between this church and their church. More significantly, the children learn of the cultural differences between blacks and whites in Maycomb. There are some who don't approve of Cal bringing two white children to the First Purchase Church (namely, Lula) but most are completely accepting. Scout and Jem learn that most of the people at Cal's church can't read and they sing without hymnbooks.
As they are leaving the church, Scout is curious about these differences and asks Cal if she can visit her sometime at Cal's house. Cal says she is welcome any time. Another thing that the children notice is how the language is different. She notes that Cal speaks a certain way when she is with the Finches and another way when she is at church. This illustrates a cultural difference and when it comes to language, this is sometimes called "code switching." Cal tries to explain the reason for this when Jem asks why she speaks so differently at church:
"Suppose you and Scout talked colored folks' talk at home--it'd be out of place, wouldn't it? Now what if I talked white folks' talk at church, and with my neighbors? They'd think I was puttin' on airs to beat Moses."