Lula and Calpurnia have a showdown when Jem and Scout enter the church. Lula is livid that Calpurnia would bring white children into a black church. She outrightly questions Calpurnia. Calpurnia retorts by saying that the children have a right to be there. More to the point, they worship the same God. Calpurnia does not back down an inch.
Here is a quote that shows the confrontation:
I felt Calpurnia’s hand dig into my shoulder. “What you want, Lula?” she asked, in tones I had never heard her use. She spoke quietly, contemptuously.
I wants to know why you bringin‘ white chillun to nigger church."
"They’s my comp’ny," said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.
"Yeah, an‘ I reckon you’s comp’ny at the Finch house durin’ the week."
A murmur ran through the crowd. “Don’t you fret,” Calpurnia whispered to me, but the roses on her hat trembled indignantly.
In terms of why Lula reacts this way, there are at least two reasons. First, the black community knows what is happening with Tom Robinson. He is being unjustly accused of raping and beating a white woman. Therefore, the blacks see the racism and hate.
Second, even without Tom Robinson's predicament, the blacks feel resentment over years of mistreatment and racism. Furthermore, one of the few places they have of their own is church. Therefore, to bring white people into it is to transgress their sacred space. Anger is the outcome.