Good verses evil is one of the underlying themes of many epic tales. It's a successful theme because it is something that every human relates to. As writers we follow the hero through our imagination on his journey through the valleys of despair and at other times climb with him to the mountaintops where the sun breaks through the stormy sky. In fact, as writers, it's fun to place the evil castle on the sunny mountain top just to mislead readers enough to catch them unawares.
In real life, Satan does the same thing. He tries to mislead. To avoid his deception, we as Christians need to walk through this world trained in the righteousness of God. If we become "dull of hearing" we will soon find ourselves not as discerning as we need to be but more gullible—like a babe. In fact, if our senses are not trained to discern, we might believe the castle at the top of the mountain is good just because the setting is sunshine and roses.
Just like our favorite characters who learn to discern good verses evil throughout the hero's journey, within the spiritual realm we are each on a quest. As part of the body of Christ, we are responsible to do our part—we are to train. How? Through our practice. "Solid food is for the mature, who because of practice have their senses trained to discern good and evil" (Heb. 5:14).
When writing biblical speculative fiction, these are the same principles that make a plot not only interesting, but something the reader can identify with. A flawed character will grow beyond where he starts in the story as a writer takes these principles and applies them to their characters as they travel into the valley or climb to the mountain top. Characters become more interesting as they grow throughout the story with their senses honed to see good and evil because they have been trained with biblical principles put into practice. And through their examples, these fictional lives can make a difference in the reader's lives. It's fiction with a purpose.