At one point in my career, I published two books with an epub. I also had an agent, later. Both of those experiences taught me a lot about writing for the Christian market.
I learned, from both of these places, that to make it in the Christian market, your book can't preach. It has to be what I call 'Christian lite' to make it, or people will put it down.
Why? Even if the readers are all Christian, there are different beliefs and denominations. You don't want to offend anyone. You also want to bring in the non-Christian sector, to read your book.
Here's an example. I had a part of one of my books that mentioned a baptism. It was a full baptism, in a 'pool' font. Now when I was baptized, the minister sprinkled water on my head. So I had to do research on a full immersion. I learned a lot. I gave it to the editor, and they nixed it right away. Not every church baptizes the same way, and not everyone believes the person should wait to be baptized. I took the scene out and it was received better.
I've also learned not to have the characters judge others or push their religion on the non-believers. If they do, the non-believing characters get upset, just like in real life, unless it makes sense in context. That's not easy to do, but it makes it easier to read.
However, I do know other Christian authors who think their fiction should always be preachy. They have to tell the reader to follow their Christian beliefs, or they're doomed. I'm not so sure that's the way to go, though. It's not good to turn off a reader.
If you're going to write Christian fiction, I'd suggest keeping it light, and focus on the relationships instead of the preaching. It'll be just as effective as beating the theme over the reader's head because you may get the reader to investigate more.
Have a wonderful week!