You can tell it's fiction mainly because it contains elements that are clearly products of the author's imagination, like the unique portrayal of the Trinity. Nonfiction would be based on real events or facts.
Well, 'the shack book fiction' is a really interesting piece. It's about a man named Mack who has endured a great tragedy. He receives an invitation to the shack where his daughter was murdered. There, he meets some rather unusual characters who represent different aspects of the divine. Through his interactions with them, he begins to understand love, pain, and forgiveness in new ways.
The main character in 'the shack book fiction' is Mack. He is a man dealing with a lot of pain and guilt after the loss of his daughter. There are also some other characters that he meets at the shack which represent different spiritual elements, but Mack is the one whose journey we follow throughout the story.