I didn't think it was possible, but it's true--I've managed to find a nineteenth-century novel worse than William Sewell's Hawkstone (1845). Allow me to explain the magnitude of this discovery. If conservatives are liberals who have been mugged, then Sewell demonstrates that a rabid Victorian anti-Catholic is a Tractarian who has seen several co-religionists take "the road to Rome." Hawkstone, his heavy-breathing, wetly fulminating and in general sludgily grotesque attack on all things having to do with Catholics, Low Church evangelicals and trade unionists, is a fine example of the depths to which Victorian didactic prose was capable of sinking. Sewell's idea of dialogue, for example, consists of one person asking questions and the other responding in page-long spurts of Anglican theology. The plot, which rambles on for over 800 pages, manages to climax with the main villain being eaten by rats (ewww). Sewell understands neither character development nor pacing. The whole thing is a turgid triple-decker mess.
To find an even less competent didactic novel, I had to cross the Atlantic. The winner is...Julia McNair Wright's Almost a Priest. Wright, a postbellum nativist, recycles a number of standard anti-Catholic plots, most notably that of the heiress "seduced" by the Church for her money. But to call her narrative technique "plotting" would require more Christian charity than I currently possess. Instead, Wright offers the reader a loosely concatenated run of scenes, all stultified instead of animated by wooden dialogue. Instead of characterization, Wright supplies stereotypical boilerplate. Even the illustrations are incompetent. (The illustrator never mastered facial proportions; in one case, a "beautiful" young woman comes out looking like Bob Hope with heavy eyebrows.) There's literally nothing positive to be said about the book. It's a truly amazing achievement!