I’m a Christian. The very core of that means I believe, with everything I am, that God is beautiful. That everything clean and pure that I love and admire in the world is a reflection of him. Or, stated differently, anything that reflects his beauty is what *is* pure and admirable. Everything that I love is found, it is fulfilled, it is satisfied, in him.
Stories, in their own small way, have a chance to reflect the beauty of God in much the same way the world does. Everything good in a story: a beloved character, a satisfying relationship, a victory against a villain. All of these things have the potential to reflect or give a taste of the beauty of God.
I think, in a way, whenever those reflections are tainted, twisted to the world’s standards, it takes away that beauty. For those of us who see and feel it, it is like having something good and greatly desired, something longed for, violently ripped away or mutated into something ugly.
It would follow, then, that "Christian" fiction would be something that, no matter the plot or characters, would nurture that taste… create that desire and longing, rather than kill it. That isn't the same as omitting bad choices, writing just fluff, or overt Christian language, of course. It's something that shows the desirability of a good relationship between family, husbands and wives, friends. The worth of good choices even when they are hard. Hope instead of hopelessness. Things that reflect God's faithfulness, his holiness, his justice, his love.
We live surrounded by corruption, apathetic towards good. Good fiction, in my opinion, creates that thirst for what is good. C.S. Lewis was particularly gifted in this.