Calpurnia takes Jem and Scout to her church in Chapter 12. Atticus had gone to an emergency session of the state legistlaure, and so would be away from home for two weeks.
Calpurnia has some reservations about taking the two to her church, mainly because of behavior issues. However, with promises to not embarrass her, the two attend the First Purchase African A.M.E. Church with Calpurnia.
There, Jem and Scout experience reverse discrimination for the first time. Lula berates Calpurnia for bringing "white chillun" to a black church. They have their church, we have ours," she states. This is an example of the reinforcement of segregation from both sides of the race issue.
When Calpurnia takes the children to church with her, in addition to their experiencing racism "from the other side of the fence," the children perceive how protective the church members are of one another as Lulu sees the arrival of Jem and Scout as an invasion. Reverend Sykes also demonstrates his true Christian charity in not using his pulpit to rail against the injustices against Tom Robinson unlike some so-called preachers in contemporary times who have denounced America and certain groups of Americans.
Further, Jem and Scout are witness to the poverty of the church members who cannot afford songbooks; yet, they all find something to contribute to their "brother" and his family in need. Indeed, there is a love that permeates this humble church, a sense of Christian charity that is sadly lacking when, in a later chapter, Mrs. Merriweather has the temerity to derogate Atticus in his own home before his sister Alexandra, who is the hostess of the Missionary Tea, not to mention her demeaning comments about Maycomb's black community and her maid after this sanctimonious hypocrite has only finished talking about the wonderful missionary work done in Africa, yet she denegrates the African-Americans in her own town.
Calpurnia takes Jem and Scout to her church in Chapter Twelve of To Kill a Mockingbird. The night before the service, Calpurnia bathes both Jem and Scout, and she reviews their clothing (a suit for Jem and a dress with petticoats and a pink sash for Scout) and treats the material with starch the morning of. Jem wryly comments that with all this fuss, "It's like we were goin' to Mardi Gras."
The church that Calpurnia takes them to is First Purchase African M.E. Church in the Quarters; the building was purchased with the first earnings of freed slaves and contains the only steeple and bell in all of Maycomb.
When the trio first arrives at the church, the men take off their hats and the women cross their arms, which Scout describes as "weekday gestures of respectful attention." One woman, Lula, is not amused that Calpurnia has brought white children to a black church. Jem and Scout want to leave to avoid causing trouble, but another church member, Zeebo, assures them that they are welcome there.
Scout and Jem are surprised at the lack of decor, hymn-books, or traditional instruments for church music. Despite these superficial differences, the sermon is much the same as the one given at their own church... with the exception of the calling out of individuals who have sinned in some capacity. The service closes with continued money-collecting on the behalf of Helen Robinson, who cannot find work due to people's skepticism about her as the wife of Tom Robinson.
This experience is significant because, as Scout puts it, the children discover "[t]hat Calpurnia led a modest double life... a separate existence outside our household... having command of two languages." This recognition of Calpurnia's private life and multidimensional existence as a human being is critical; she is no longer viewed by Scout as merely the black woman who works in the Finch household. She is a person beyond her housekeeping and the color of her skin, one with very real desires, needs, principles, cultural values, etc.
They were taken to church in chapter 12 and within this time, they were exposed to more African Americans. When they entered the church they were met with a rude African American and Calpurnia changed her talk into one of "her people" even though she knew how to speak English very well.