I've had a few postmill characters, and in one of my stories--the only "End-Times" novel I've written--there's even a postmill movement called Millennium Now. Unfortunately, MN was split: the ordinary people who went around evangelizing in its name had a very silly, simplistic view of its teachings, while the actual teaching tended to be known only to the more scholarly. I liked the real version, though only the silly version actually cropped up much in the story. (The token brain stated clearly that the zealots were ill-informed and easy to beat in an argument, while the real MN people were far better informed and much more formidable in debate.)
Similarly, a proposed alternative history series features a postmill inventor in the late nineteenth and early twentieth centuries.
But in general we don't have characters who are that far removed from our own theology. In my upcoming series "The League of Superheroes," I do have a couple Catholic characters and an Episcopalian who are all amill, but eschatology doesn't really come into the stories much. The theological differences are somewhat important--the Catholics see things in a way that most Protestants wouldn't, for example, and a Baptist character is likewise fairly typical for that group except that he gets on well with the Catholics and doesn't serously try to convert them.
But having characters from other denominations and doctrinal positions is one thing; do we actually represent another view as fact, at least within a story world? I did so in my short story "One Taken, the Other Left" in the anthology Light at the Edge of Darkness, where I posit an odd twist to the usual pre-trib Rapture scenario. (One reviewer accused me of putting "bad theology" in the Lord's mouth, but he couldn't be bothered to explain what was bad about it. From my knowledge of the reviewer in question, I would put my knowledge of theology and the Bible up against his any time. And for the record, the points attributed to the Lord in the story are actually quite standard theology; I just applied them where most others would for some reason make an exception.)
But the question remains: do our settings have to reflect our doctrinal positions? I don't see that they must, so long as the views are either standard doctrine (as postmillennialism is in some circles) or minor tweaks on them. This is speculative fiction, after all, and weirder matters arise. (It's worth noting that Chris Walley's The Shadow and Night doesn't have any alien races, which at least eliminates that problem.)
1 year ago