I don't think they're bad because of the religious aspects; though I'm not myself a believer, I have a healthy respect for other peoples' faith. Besides, if I can suspend disbelief for Dark Knight, I think I can manage a few demons and angels.The first comment sums up most of the rest:
The problem is, the writing is dreadful. The Left Behind series reads like it was written by a fourteen year old B student with a HUGE crush on Jesus Christ. To call the characters cardboard cutouts would be an insult to paper dolls, which are vastly more realistic than anything created by Messrs Lehaye and Jenkins. The dialogue reads like it's been triple-starched. And the plot belongs in a churchyard.
There's no reason this should be so; religious faith is one of the great human dramas. Nor is it that they are pitched to a general audience; there are a lot of great mass-market storytellers. So why haven't better writers emerged in this genre?
The Christian Market has demonstrated that they'll buy pretty much anything based on worldview rather than on quality.While true, I think this only explains why there is lots of cruft littered around, not why even the best selling books are horrible. Maybe anyone can move books by slapping Jesus on the cover and that results in lots of bad writing, but that's not the same as describing why there isn't any good writing. If you have twenty books with the same Rah-Rah, Go Jesus! world view, at least one ought to be decently written, and that one ought to attract at least a little more attention than the others. After all, religion is "one of the great human dramas." It should attract good writers because it's good material. There's a lot of potential there.
I think the reason is more closely tied to the adjective the commenter left out. These books are not appealing to Christians in general. They're appealing to evangelical and Pentecostal Protestants.
You don't see a lot of good evangelical fiction for the same reason you don't see a lot of love stories published in which boy meets girl, boy and girl get married, and boy and girl live happily ever after. That's the way it's supposed to go. In a way, that's the narrative status quo. And it's boring. You need to do things differently for that story to be interesting.
But I don't think it's really possible to shake up the narrative in Christian literature that much and have it remain appealing to the megachurch crowd that you need to market it to, because that's not a culture that values a lot of creative input when it comes to religious themes. The stories are all set already. A said B at location C, and Lo! after D days E happened. If you do X, then Y will happen to you. There's no room for E' or for Z happening instead of Y. These things are all already established and immutable in the minds of the audience, so the author doesn't have a lot of room to do interesting things with them. There's no room for speculation or personal interpretation. Furthermore the inclusion of syntactic elements like a wizard or a talking animal are going to draw protests even if the semantics of the story remains thoroughly Christian, so there isn't as much wiggle room in the presentation either.
I think this necessary distinction between Christian and evangelical Christian are why the examples left in the comments of good Christian fiction (C.S. Lewis, J.R.R. Tolkien, Madelein L'Engle, and I would add G.K.Chesterton) consist of three Catholics and an Episcopalian. Both of those traditions are far, far less literal. There's much more room for an author to draw their own theses. But these stories are never going to appeal to Michael Megachurch or Polly Pentecostal specifically because the author gets to present their own vision, and not the One Established Truth they are looking for.
I also think this is why a lot of fiction I like which takes on religious themes are written by, shall we say, unbelievers. There is good fiction being written about religion, but as often as not we classify it as irreligious and exclude it from consideration. Philip Pullman, Tom Robbins and Robert Heinlein all wrote about the great human drama of religion, and I think they're good at it for the same reasons that good love stories are written by people who've had their hearts broken and good war stories are written by soldiers who didn't particularly care for war. I'm sure there were plenty of Russian novels being written about how great Communism was, and I'm also sure they were all horrible compared to Solzhenitsyn. Sometimes outsiders and dissidents can see things more clearly and present things more passionately than those on the inside.
Addendum: One of McArdle's commenters, Elizabeth, mentioned that if you want good apocalyptic fiction you should forget about LaHaye and Jenkins and pick up Pratchett and Gaiman's Good Omens: The Nicce and Accurate Prophecies of Agnes Nutter, Witch. I whole heartedly agree, although it would make most Left Behind readers' heads explode. This passage has stuck with me since I first read it when I was twelve or thirteen:
God does not play dice with the universe; He plays an ineffable game of his own devising, which might be compared, from the perspective of any of the other players, to being involved in an obscure and complex version of poker in a pitch dark room, with blank cards, for infinite stakes, with a dealer who won't tell you the rules, and who smiles all the time.I also like Crowley's use of traffic jams to sew evil in the hearts of men. I think about that every time I get on 495.