The whole story is about how two men establish a close relationship because of their experience with a television show about cathedrals built in the Middle Ages. Robert can hear what the announcer is saying about these cathedrals but cannot see them, and he acknowledges that he really doesn't know what a cathedral is because he has never been able to see one. The narrator does his best to describe a cathedral to the blind man but fails in the attempt, partly because of his limited vocabulary and partly because of his ignorance about the subject.
The subject of cathedrals naturally leads to the subject of religion, and the two men have an extremely interesting exchange.
"That's all right, bub," the blind man said. "Hey, listen. I hope you don't mind my asking you. Can i ask you something? Let me ask you a simple question, yes or no. I'm just curious and there's no offense. You're my host. But let me ask if you are in any way religious? You don't mind my asking?"
I shook my head. He couldn't see that, though. A wink is the same as a nod to a blind man. "I guess I don't believe in it. In anything. Sometimes it's hard. You know what I'm saying?"
"Sure, I do," he said.
"Cathedral" is the right title for this story because the subject rather mysteriously brings these two strangers close together exchanging truths about themselves: what it feels like to be blind, and what it feels like to believe in nothing. They end up with the narrator trying to draw a cathedral on a flattened-out paper bag with a ballpoint pen, while the blind man keeps his hand on the narrator's moving hand and is thereby able to visualize approximately what a cathedral looks like. This scene dramatizes modern man's loss of religious faith and his attempt to capture the faintest glimmer of it from a drawing copied from a picture on a television screen.
Many of Carver's stories reflect the emptiness and meaninglessness of modern life, as well as the drinking and other substance abuse that are a desperate attempt to escape from this frightening reality. The feelings and insights experienced by the narrator will change his life very little. He realizes that:
"In those olden days, when they built cathedrals, men wanted to be close to God. In those olden days, God was an important part of everyone's life. You could tell this from their cathedral-building."
But then when he finds he can't describe them to Robert, he confesses:
"The truth is, cathedrals don't mean anything special to me. Nothing. Cathedrals. They're something to look at on late-night TV. That's all they are."
He can attempt to explain what they look like, but he can't begin to express the powerful emotions that went into their creation.
Perhaps for a few moments these two lonely men are "close to God" when they cooperate, as did the craftemen of the Middle Ages, in creating their own cathedral with a ballpoint pen on a grocery bag. The two of them together are doing something that neither could do separately. The narrator is trying to show Robert what a cathedral looks like, but it is Robert who is guiding him.
"That's right. That's good," he said. "Sure. You got it, bub, I can tell. You didn't think you could. But you can, can't you? You're cooking with gas now. You know what I'm saying? We're going to really have us something here in a minute."