It's a sci-fi/fantasy story that at one point has a husband and wife traveling into Hell to save someone/something. I want to say that one or both of them are wizards. Anyway, they are warned that if things should go south, the Christian God and his angels have no dominion there. Hijinks ensue, and they are trapped by an army of demons. Ultimately, the cavalry arrives in the form of several non-Christian deities (I want to say Athena, Thor and Quetzlcoatl).