I just read a line by Flannery O’Connor in Mystery and Manners that explains why this is so: “what is written to edify usually ends by amusing.” The word “amusing” is what triggered the realization. Humor is often produced by incongruity, contradiction, and paradox. The fool is comic because man is not supposed to be foolish. The wise man is good for a platitude, the idiot for a laugh.
Writers get in a trap when they set out to write what is edifying and seek to avoid the failures, the falls, the disappointments, the crises, the impieties, the sins inherent to life. To the extent that these things are thought unedifying and thus inadequately represented in an effort to be edifying, the writer creates a tale that does’t square and so inadvertently creates a joke instead of a convincing or compelling story. And accidental jokes are only accidentally edifying. Usually they are merely, as O’Connor says, amusing.
I think this is true when writers draw characters who are overly pietistic as well. “A writer writes about what he is able to make believable,” says O’Connor. We live in an age when religion is often seen as threatening or absurd. Overly pietistic characters work well as terrorists (think Islamic militants and abortion-clinic shooters) but as good-natured characters they can come off as boobs unless the character picture is full orbed — which is to say, inclusive of their faults and failings. And even then, watch out.
It is far better to follow Montaigne’s approach: “I do not teach, I relate.” Tell a story, tell it well, and let it be edifying in and of itself. O’Connor again: “The artist has his hands full and does his duty if he attends to his art. He can safely leave evangelizing to the evangelists.” And the comedy to the comedians.