Regardless if the book is about the Catholic religion, or any other religion for that matter, the first thing that needs to be done before you start writing your book is to identify the reason for writing it.
Finding the purpose should be the first step any author takes before he/she starts writing a book. Once you have a reason by design you identified a need which in turn recognizes a market.
Unlike writing a diary for yourself a book is something you're expecting and hoping others will read, and want to read. So as I said, the first thing is to discover the need, this will help you identify the market and then you research the market to find your particular niche.
What you need to be working on very hard in the beginning is on the idea and the message you want to convey. What is it you want to change? How do you want to inspire. If you're applying to a religious section you need to know your subject and you need to know it well because usually people who are devout in their religion know their religion as well as you, if not more.
So expect resistance no matter from what angle you seek to improve. Taking that into consideration, rather than dismiss, build up. Remember when you're writing a book that deals with religion, you are touching a sacred spot in people's hearts a raw nerve to some. Try not to be offensive because you will shut your audience out.
Once you have written the book you will know where you should be promoting it (you have already done the research) and where you should be distributing it. Before we get to promoting and distributing, we need to get the book printed and this is where TheBookPatch comes in.
You need to make sure that the cover reflects the ideas in the book. Remember that people that see the cover have not read the book yet, so the cover needs to peek their curiosity and promise something intriguing but not tell the story that's inside.
Finally, the text itself should be set up in a way that is easy to read, what that means is that it is formatted in the style that has been done in similar books in the past. You don't want to take away from your story and distract from it via innovative design that serves no purpose, keep it simple.
And then again, once the manuscript is in book form you can upload it as a PDF, upload your cover and you're off to the races with a book that has been pre-thought for its market and designed for the people who are interested in the subject and conveys an idea that is worthy of the printed book.