This is a thoughtful and question and one that probably can't be answered just from the pages of the novel.
Calpurnia, the Finch's housekeeper, is probably one of the best educated and well-paid African-American women in Maycomb. She is one of the few members of the First Purchase Church congregation who can read and write, and she has seen to it that her son, Zeebo, is also literate. She grew up near Finch's Landing, and she worked for Atticus's father before moving to Maycomb when Atticus was married. Cal is entrusted by Atticus of bringing up his children following the death of his wife, and Atticus tells his sister that Cal has
"... never let them get away with anything, she's never indulged them the way most colored nurses do. She tried to bring them up according to her lights, and Cal's lights are pretty good... and another thing, the children love her."
Calpurnia seemingly exists happily in both the white and black worlds of Maycomb, leading "a modest double life" that Scout had never considered before. She was taught to read primarily through "Blackstone's Commentaries," and Scout realizes this is why Cal speaks such proper English in the Finch household. But when Cal returns home, she speaks "colored-folks' talk," a fact that puzzles Scout. Cal is forced to walk a fine line between her employers and her friends, and she explains that if she spoke
"... white folks' talk at church and with my neighbors... They'd think I was puttin' on airs to beat Moses."
She considers it "unladylike" to "tell all you know," and that her friends may not take kindly to someone who pretensiously exhibits
"... knowin' more than they do. It aggravates 'em. You're not gonna change any of 'em by talkin' right, they've got to want to learn for themselves, and when they don't want to learn, there's nothing you can do but keep your mouth shut or talk their language."
Cal's seemingly unorthodox manner seems to work, however. The congregation respectfully honors Jem's and Scout's appearance when they enter with Cal; and when she is confronted at the church by Lula, who objects to the presence of the white children at her black church, the other members--Cal's friends--quickly crowd her out.