If you are referring to the time when Calpurnia takes Jem and Scout to her church, it occurs in Chapter 12. The church is called First Purchase African M.E. Zion Church.
First Purchase African M.E. Church was in the Quarters outside the southern town limits, across the old sawmill tracks. It was an ancient paint-peeled frame building, the only church in Maycomb with a steeple and bell, called First Purchase because it was paid for from the first earnings of freed slaves. Negroes worshiped in it on Sundays and white men gambled in it on weekdays.
Scout gives a fairly detailed description because this is a new and interesting experience. When they arrive, they are accosted by Lula who asks Calpurnia why she brought white children to their church. Calpurnia responds that it is the same God in each church. This is the first time, in the novel, that the children feel like social outcasts. Prior to this, they were beginning to understand the social divisions of Maycomb and the outsider statuses of people like Boo, Tom Robinson, and the Ewells. But this is significant because Scout and Jem experience what it is like to be outsiders.