I’ve started and stopped a lot of books. Sometimes it’s easier (and better!) to begin with a new one than to “fix” the old one.
I sugggest you begin with the idea of telling a good story. Don’t think about teaching religious education or making a “Catholic” point, but let who you are – a Catholic – be reflected in what you write. Write what you know, right?
Some authors like a complete outline before they begin. Others tend to wing it more. I’m more a winger. I have a character I like, a plot that seems interesting to me, a problem that the main character will soon have to begin solving or dealing with, and an idea of how the book will end.
Whatever method works best for you is just fine.
The real key is to write a set number of words every day, six days a week. Even if that’s only 250 or 300 words, it adds up quickly over the weeks and months.
The secret is there is no secret. It takes a commitment and work. But being able to use your God-given gift of creativity is a real joy.