The central idea of "Cathedral" involves the epiphany the narrator experiences. You might think of it in this way, if the the blindman 'opens' the narrator's eyes so to speak, then what significance does this hold for the narrator.
.In his attempt to explain what a cathedral looks like to Robert, the narrator cannot find words. However, Robert encourages the narrator to help him draw a cathedral to get a better idea of what it means. While doing this, the narrator experiences his epiphany. When he closes his eyes, he and the blindman create something together.
This epiphany relates to many of the themes inherent in the story (see the second link below). The narrator realizes how shallow and isolated his life has been. Up until this point, there is nothing significant about him - not as a man, a husband, or a character. Yet, one could argue that the narrator has forged a stronger bond with the blind man by this simple task of drawing than he has forged with anyone else in his life.
The narrator also realizes the power of his imagination and the ability to create something. He has been awakened to the depth that life and experiences can offer. No longer does he have to just watch his life unfold; he can help shape it. Of course, the irony is clearly evident: the narrator learns how to shown something from a man who cannot see.
To provide you with a few different themes in this particular format:
If humans close their eyes, then they can open up their imagination.
** the narrator learns to close his eyes to what he sees in front of him and allows his mind to see beyond that as he is guided in his drawing of the cathedral.
If humans close their eyes, then they can open up their hearts.
** by closing his eyes and allowing his hand to be controlled, the narrator learns to connect with his visitor.
If humans withhold judgement, they can find inner peace.
** the narrator, when he finally lets go of the judgements he had about the visitor's condition is able to relax and let go of his own struggles