In layman's lore, the surest sign of sanity is the ability to laugh at oneself. That ability--and, occasionally, that sign--was, until very recently, sadly lacking in the institutions of organized religion. But the pendulum has swung with a vengeance, as witness the publication last year of St. Fidgets and now of Bittersweet Grace. The latter is a collection of contemporary religious satire and spares not dogma, institution, cleric nor layman. Not even Himself escapes altogether. some of the selections are classics--e.g., archiconoclast Mencken's ""Holy Clerks"" and Power's ""Prince of Darkness""; others should be: Werner Pelz's ""And Cried Bitterly"" and Peter Malton's ""Instant Baptism."" A few go rather wide of the mark, probably because those attempt to satirize situations the reality of which is itself satire. But the collection as a whole is sheer delight, a bonanza for summer readers.