I agree with mckapen1's statement that the central fear in Harper Lee's novel To Kill a Mockingbird is a fear of the "dark" -- meaning. of course, the white people's fear of black people. The black people are afraid, too, in the novel; no black person will walk by the Radley house at night without whistling, I believe, and Tom Robinson claims on the witness stand that he was more terrified than anything when the Ewell daughter made advances toward him.
The final chapters of the novel address this topic in a fairly direct way. A friend scares Jem and Scout on their way to the Halloween festival (here, there's nothing to fear), but on their way home, when they think it's their friend playing another prank on them, they're really being stalked by an outraged man with a knife (here, there really is something to fear).
Is FDR's famous phrase "true," then? In some cases, at least, yes. Maybe not always.
In the book "To Kill a Mockingbird" Harper Lee is trying to convey a message about prejudice. Prejudice comes from lack of understanding about others and the result is fear. For example: In the book the reader learns about Boo Radley. Boo lives in a house that is run down and worn. The yard is overgrown with weeds. The house is kind of spooky looking. Rumors travel through the town about its occupants. There is knowledge that at one time one of the sons had stabbed someone in the leg in the family. No one ever sees the son, Boo. These things give way to a fear of the Radleys. They become the town boogie man and haunted house.
Racism is a strong issue in the story. Calpurnia takes the children to church with her in the black community. The children are raised to respect all people and to treat them with respect. They do not have a fear of black people. Instead they are as comfortable in the black community with Calpurnia as the white community. However, many people in the town are the opposite. They have created fears for themselves when there are none. It is their fear of black men raping the white woman that prevents the jury from seeing Tom's innocence.