The Christian Writer’s Manual of Style edited by Robert Hudson is my second most used reference book. Since so many of my clients write for a Christian audience, I use it to make sure I help them follow the rules the big publishing houses are following.
One of the sections I use most often is “Capitalization of Common Religious Terms.” It helps me remember when we capitalize gospel and when we don’t (hint: we do when it’s “the Gospels” referring to the collective title of the four gospel accounts as a whole, but we don’t when it’s just “the gospel message” or even “the gospel of Matthew”). If you write for a primarily Christian audience, I recommend The Christian Writers’ Manual of Style. (affiliate link)
(Want to read more quotes from my favorite books for writers?)