Spoiler for Steven Brust's _To Reign In Hell_. . . .

It has never before occurred to me to consider the default mode of Christian dualism as being an old chestnut of a logic problem:

There are two doors, one leading on towards the goal, the other heading for certain deat^H^H^Hamnation. The guardian of one door tells the truth, and the guardian of the other always lies.

The tricky bit is figuring out which is which. . . .

I'd steal this for my own cosmology, but I don't entirely know where to put it. Hail Eris.