Calpurnia is a rarity in Maycomb: She is a black woman who can read and write--one of the few members of the First Purchase Church congregation who is literate--and who is able to supervise the Finch house and children with the full confidence of her employer. It is Calpurnia who teaches Scout about how to treat a house guest when she chastizes Atticus's daughter about her behavior when Walter Cunningham Jr. is invited for lunch.
"... anybody sets foot in this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty!" (Chapter 3)
The children's trip to Cal's church teaches them that there is little difference--aside from the extreme poverty--between the churchgoers at their own place of worship. Cal's friends treat the children with friendliness and respect, and
Jem and I had heard the same sermon Sunday after Sunday... (Chapter 12)
That same Sunday, the children learn another lesson from Cal: that she secretly spoke differently in the Finch house than she did with her friends.
That Calpurnia led a modest double life never dawned on me. (Chapter 12)
Cal told Scout that it's "not ladylike" going "around knowin' more than they [her black friends] do"--and it is a lesson that Scout takes to heart after witnessing the bad behavior by the supposedly devout ladies of the missionary circle.