In my humble opinion, I don't think "Christian" is a real genre at all. (I say this as a former Evangelical who recently converted to Catholicism, I having grown up on VeggieTales. )

I would say that there are no "Christian" books, just like there are no "superhero" movies. There are only comedies, dramas, action-movies, crime films, etc. which happen to feature superheroes, or in this case, only books of different genres which happen to feature Christians as characters.

This doesn't mean that you can't write a story where the protagonists are Christians and the subject of the story is their faith. I would love to see more books like that. The key thing to remember is that it's too easy to turn such a story into a propaganda piece.

To illustrate what I mean, I once was acquainted with an indie writer who wrote these self-published science-fiction novels about a Christian who meets a bunch of aliens and is recruited by the good aliens to be a missionary reaching out to the bad aliens. Aside from certain problematic theological stances my acquaintance took in her books, there was one chief, glaring flaw that made me realize just how silly the whole series was: All of the Christian characters are so. Darn. Absolutely. Perfect.

The hero never dealt with any serious temptation to sin, all the other Christian characters had no real flaws, the entire race of the good aliens was (by authorial fiat) incapable of being sinful, and miracles and what not were frequently utilized as deus ex machinas. In hindsight, the only reason I particularly liked those books as a kid was that they reinforced existing prejudices integral to my worldview, which amounted to, in sum, that all Christians are nice and anyone who's not a Christian is that way they are because they're stupid, evil, jerks, or crazy.

Now, let me be clear; this problem is precisely the reverse when a lot of secular authors write about Christians, and it incredibly annoys me when anyone does this sort of thing. I have no sympathy for uniformed world-building and characterization of any variety.

But remember: If you're going to write a story about Christians, don't make it into what might be called a "Christian book." Make it into a story of a more exact, established genre and throw a Christian into the mix, while avoiding the standard pitfalls one might run into with other such stories. Give all your characters interesting motivations and genuine flaws, whether those characters are Christians or not. Make them undertake a specific actions to achieve a specific goal.

To be brief, it's okay to write a story about a Christian character. Just make sure that it's a good story first and foremost.