For all of the time I spend writing Christian fiction, I don't believe I've ever spoken about it specifically, and I think it's about time.
As with all fiction, the characters, setting, dialogue and so forth must be well-written; this is a given for any fiction, but there are some things to remember when writing Christian fiction. Here is a short list of a few things I try to remember when I'm writing Christian fiction:
The main character must grow and change, but this growth needs to have some sort of spiritual hook. This means the character may have a difficult time praying to God or going to church or living a Christ centered life away from it.
And while Christian fiction may delve into heavy topics such as spousal abuse, childhood neglect, or drinking problems, etc., the language expressing the difficulties must be handled with kid gloves. The topic itself must not be explicit, rather inferred, and this creates some difficulty for authors who want to share the truth of it.
Christian fiction can include scripture, but the scriptures used shouldn't be too heavy handed; you don't want any preaching felt in your book, rather, you want the reader to feel hopeful for the main character as well as guilt-free concerning his/her own life.
Again, that doesn't mean that heavy topics can't be discussed in your book, but that you watch out when it comes to pointing your finger at the reader. Yes, this can happen even in a fiction book because, in Christian fiction, elements of truth and doctrine are sprinkled throughout, and you don't want the reader tossing your book aside because you've been too heavy handed.
Christian fiction can include romance and mystery, but again, the romance and mystery is written to reflect the Christian attitudes of the main character. Christian fiction is not main stream fiction for good reason.
The person picking up your book expects it to be clean and inspirational. They expect to be taken on a journey of faith, and to grow their own faith in the process. No pulling is required. Like a shepherd who leads the sheep, the reader expects to follow behind the Master.