From the other side
Why handing the story to the wrong side works on children
A story narrated by the opposition does something a story with a moral cannot: it lets a child reach the conclusion themselves, which is the only way the conclusion sticks.
Give a child a story with a lesson in it and one of two things happens. Either they miss the lesson, which is a waste, or they spot it, which is worse. A child who spots the lesson stops reading a story and starts being taught, and most children can tell the difference by about age seven.
Handing the narration to the wrong side removes the problem at the root. There is no lesson to spot, because the narrator is not qualified to give one.
The reader does the work, so the reader owns the result
When a demon explains why his job is going well, the child has to run the translation themselves. Every sentence arrives needing to be turned over. That is not a burden, it is the entertainment: it is the same pleasure as dramatic irony in a pantomime, where the audience can see what the character cannot.
A conclusion a reader assembled themselves is theirs. A conclusion handed to them belongs to the person who handed it over, and can be handed straight back at the first opportunity, usually around age fifteen.
It makes the antagonist a person
There is a practical benefit that has nothing to do with faith. A story told from the opposition's side cannot have a cardboard villain, because you are inside his head and cardboard has no inside.
Damon has a job, a boss, procedures, and opinions about all three. That is funnier than a monster and it is also more useful: a child who has spent eighty pages inside the head of someone who is wrong, and who thought he was right the whole way, has learned something durable about how being wrong actually feels from the inside. It does not feel like anything. That is the problem with it.
The same move, in audio
Bryant did this before he did it in a novel. On the Upside Down Church podcast he handed the pulpit to people who do not believe and let them preach, while he sat and listened, credited on the feed as the heretic.
It is the identical instinct: the way to make people think about a thing is not to explain it to them, it is to hand the microphone to the side that does not usually get it and let everyone hear what happens.
It is an old trick and it keeps working
This is not a new invention and nobody involved claims it is. Screwtape is the obvious one. Before that there is a long tradition of the fool who tells the truth because he is the only one permitted to, and after it a shelf of children's books narrated by the dog, the villain, the ghost.
The reason it keeps being reinvented is that it is the only reliable way to put a difficult idea in front of somebody who has decided not to be told things. That describes a great many eleven year olds, and quite a few adults.
This is one part of faith fiction told from the other side, the idea behind both books and the podcast.