Not trying to make anyone feel bad, but I cannot agree that The Shack is Christian fiction. When a work denies the atonement in any Christian or Lutheran sense, that is not Christian fiction.

When a work messes up the Incarnation, as Pastor Giese explains, that is not Christian fiction.

Two things a work must get right to be Christian are: A. the person of Christ; and B. the work of Christ. If a work gets both the person and work of Christ wrong, it is not Christian.

Rather than Christian fiction, it is a wolf in sheep's clothing, which is worse than a wolf in wolf's clothing.