After the narrator's wife falls asleep, her blind friend and he stay up with the television tuned to a random late night show which happens to have cathedrals featured. When Robert asks the narrator to describe the cathedrals, it is difficult for the narrator to do so because he lacks imagination and creativity; indeed, this deficiency may be why the narrator tells Robert that he believes in very little,
“I guess I don’t believe in it. In anything. Sometimes it’s hard..."
Nevertheless, the blind man, whose heart is warm and his mind and spirit expansive, asks the narrator to try to draw the cathedral for him so he can use his imagination and gain an understanding of its shape and size. And, after the narrator takes a ballpoint pen and begins to outline the shape of the cathedral, his creativity is tapped by his efforts and the warm encouragement of Robert. Indeed, the narrator experiences a revival of his soul and feels that something meaningfully spiritual has brought him outside himself into a communion with others,
My eyes were still closed. I was in my house. I knew that. But I didn’t feel like I was inside anything.
“It’s really something,” I said.
Clearly, the narrator has been touched by his shared artistic venture with Robert who understands the importance of things of the spirit of which the cathedral is, indeed, symbolic.
It's true the narrator is not very good at describing the cathedral to Robert. It's possible that this is because he is really high (the three characters do a tremendous amount of drinking, eating and smoking); it's also possible that the narrator isn't very "good" with words (he admits earlier that he doesn't really understand his wife's poetry). Another factor, however, has to be his apprehension about Robert and his relationship to the narrator's wife. It's clear that the wife has a strong emotional bond with Robert and that this bond has been the most enduring relationship of her life. The narrator is clearly jealous of Robert: not only is he a threat, but he is very mysterious—the narrator can't understand what it must be like to be blind and can't understand how someone could love a blind man (when he hears about Beulah, Robert's late wife, all he can think about is how it must have made no difference if she wore make up or not).
So perhaps part of his failure in describing the cathedral comes from this mistrust. However, it's also clear that the narrator comes into some self-knowledge as a result of trying to describe and draw the cathedral. Under Robert's encouragement, he goes from saying that "cathedrals don't mean anything special to me" to understanding his drawing, without opening his eyes to see it, as "really something." The narrator comes to understand (a little) what it is to be blind but also comes to understand the nature of his wife's relationship to Robert.