The following excerpt from “The Rhetoric of Fiction” explains this very well:

According to Wayne Booth in “ The Rhetoric of Fiction,” if a narrator depicts facts and opinions that do not coincide with those of the other characters or commonly held beliefs or theories of the time, then the narrator is no longer a reliable source, because their perspective is skewed to the point of being misleading. The reader quickly sheds any sympathy for this narrator, because the reader knows more than the narrator. And it’s only one of many ignorant statements littering the story. Warning: Objects in the mirror are closer than they appearįrom the beginning of the story, we can see the narrator’s insensitive opinions. What he sees and thinks cannot be reliable for the telling of this story. Best of all, he is a character in his own story, one that is often bigoted and vulgar toward others. My idea of blindness came from the movies.” Almost immediately, the narrator in “Cathedral” shows his ignorance and myopic opinions to the reader early in the story and never breaks character. In the opening paragraph, the narrator let’s the reader in on his thoughts, “And his being blind bothered me. The story of “Cathedral” is about a brief visit from the narrator’s wife’s longtime friend, an elderly blind man who recently lost his wife. Carver uses the narrator’s character flaws as an unreliable narrator in a way to create a dramatic effect that employs the reader’s perceptions about morals and propriety. Added to this is his style, a terse, minimalist prose that empowers the reader to enter the story-to invest their thoughts and opinions, sometimes testing them, like he does in “Cathedral,” which uses dramatic irony to make his narrator fallible and unreliable. His blue-collar, American themes reflected my upbringing in a Midwestern industrial city. Raymond Carver influenced my writing way back in undergrad.

Raymond carver's short story cathedral