Susan Srigley, author of Flannery O'Connor's Sacramental Art, in Mars Hill Audio Journal 73 quotes Flannery O'Connor in Mystery and Manners. This is from audio, so punctuation may not be exact.
If the writer believes that our life is and will remain essentially mysterious, if he looks upon us as beings existing in a created order to whose laws we freely respond, then what he sees on the surface will be of interest to him only as he can go through it into an experience of mystery itself. His kind of fiction will always be pushing its own limits outward towards the limits of mystery, because for this kind of writer the meaning of a story does not begin except at a depth where adequate motivation and adequate psychology and the various determinations have been exhausted. Such a writer will be interested in what we don't understand rather than what we do.