Perhaps the most common mistake I see Christians make is secularizing a Christian book to capture a market. What is secularizing a book? Using worldly terms to appeal to a non-Christian audience. Attempting to attract the unsaved by lowering biblical standards as opposed to standing on Christian principles and uplifting what the Word of God says.
A common statement I receive from fledgling authors is “The Holy Spirit told me to write this—every word of it exactly as it is.” Really? The Holy Spirit Who moved on a man of God to write “A word fitly spoken…” told you to use a minced oath or a curse word or vulgar words? These kinds of words should not be a part of any Christian’s vocabulary. I also commonly hear, “But the antagonist is the character who is using the words!” God gave us an unlimited vocabulary to paint a picture that the readers will understand without compromising Christ’s standard of excellent speech.
When the Holy Spirit moves you to expend time, energy and resources to write a Christian book, then allow the Holy Spirit to lead you to write a book that will cause the reader to desire to know the Christ of your book. Don’t let your book teeter on sensuality or any other earmark of secularism.