In To Kill a Mockingbird, what is it specifically that upsets Scout about what the churches teach?
The mature narrator (Scout) seems to be upset by the way both the African-American and white churches regard women.
In chapter 12 she is taken to Cal's church and she sees the same message being shoved down her throat:
"Bootleggers caused enough trouble in the Quarters, but women were worse. Again, as I had often met it in my own church, I was confronted with the Impurity of Women doctrine that seemed to preoccupy all clergymen."
Scout is sick of women always getting blamed for everything. It's as if men have no faults--it's all on the women. She feels like she doesn't even have a chance--and to have holy men tell her whether in a white church or a black church--is the final straw for her.
There are several references to the ways that women are viewed in light of religion in To Kill a Mockingbird. One day, Scout and Miss Maudie are talking about the "foot-washing Baptists." Miss Maudie tells Scout that foot-washing (or Primitive) Baptists "think women are a sin by definition" (Chapter 5).
When Scout visits Calpurnia's church, Reverend Sykes preaches on the topic of sin. Scout observes that the content of his sermon is similar to many others she has heard before:
... he warned his flock against the evils of heady brews, gambling, and strange women. Bootleggers caused enough trouble in the Quarters, but women were worse. Again, as I had often met it in my own church, I was confronted with the Impurity of Women doctrine that seemed to preoccupy all clergymen (Chapter 12).
Scout is puzzled by this preoccupation with the idea that women are sinful. She does not express that she is particularly upset by this idea or even that she disagrees with it. She simply does not understand why it is such an issue.
This Christian idea of women being sinful originates with the book of Genesis in the Bible. Supposedly, Eve was tempted by Satan in the form of a serpent and she ate the forbidden fruit. She then convinced Adam to eat the fruit. This became basis for the idea of the specific sinfulness of womankind that some denominations of Christianity believe in.