Make a list of Christian, religious or Bible-based themes that are truly of interest and important to you. After making your list, look it over and choose a theme that you feel others will benefit from the most. Visit the library for books on your topic to start your reseaarch. Make sure to take good notes. When you're ready to write, create a chapter outline.
Next write the first draft. When finished, put it aside for about 2 weeks and then read it without making corrections.
After the read-through, start revising, next edit your draft and then proofread it. Although expense, if you can afford it, get a professional editor and proofreader. Finally, buy 'Writer's Market' resource for writers to find an agent and/or publisher (or your could self-publish). Finally, get the word out by marketing your book.
PS: Even though there are atheist, the question is not a mistake, as you have the right to ask any question that you choose. Do not let others restrict your choices. You can respect atheists, but you must also respect yourself and if you want to help other Christians, which is your audience, just as some write mystery books and not romance to target their audience. Do not be afraid to voice your own opnion or speak up.
.After all, atheists have the right to speak up and they should not try to take away that right from Christians, especially since they know how bad it feels to be ostracized. Otherwise, you allow them to do to you, what they don't want done to them, to be hushed up.
So if you want to write a religious book, wite it. Atheist are free to write about their beliefs, too. Besides, you cannot please everybody,nor can they.