What does Lula and Calpurnia's interaction suggest about racial tensions in Maycomb County?

The interaction between Lula and Calpurnia is an important part of the book and shows an important point about race.
A bit of context is necessary. In chapter 12, Calpurnia takes Jem and Scout to church with her. There, the children meet Lula. Lula, rather than being friendly, becomes angry that white children are in their black church. When Calpurnia sees this, she and Lula have a conflict. Here is a quote that captures the tone:

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.”

The dialogue continues a bit, and the verbal fight escalates. What this shows is that racism exists all over Maycomb. Prejudice exists in every context. That said, it is important to note that Lula is an exception. The others in the church were very gracious and welcoming of the children. Zebo is a prime example of warmth. Here is a quote that shows this point:

One of them stepped from the crowd. It was Zeebo, the garbage collector. “Mister Jem,” he said, “we’re mighty glad to have you all here. Don’t pay no ‘tention to Lula, she’s contentious because Reverend Sykes threatened to church her. She’s a troublemaker from way back, got fancy ideas an’ haughty ways—we’re mighty glad to have you all.”

In Chapter 12, Calpurnia takes Jem and Scout to Sunday service at First Purchase African M.E. Church. When they enter the church, a woman named Lula notices the white children and says to Calpurnia, "I wants to know why you bringin' white chillun to nigger church" (74). Calpurnia tells Lula they are her company, and Lula says, "Yeah, an' I reckon you's comp'ny at the Finch house during' the week" (74). Lula tells Calpurnia she has no business bringing white children into their African American church. When Lula says, "they got their church, we got our'n. It is our church, ain't it, Miss Cal?," Cal says, "It's the same God, ain't it?" (74).
Lula and Cal's controversial interaction suggests the African American community is upset with the white citizens of Maycomb. Lula feels white people do not have the right to enter an African American church because she resents being segregated in the white community.