Yes, it is formula writing—that is, the guidelines are well defined, but you have latitude within those guidelines.
You can choose to write in first person or third. You can build around an anecdote from which to draw a spiritual lesson; you can retell a Bible story, focus on a familiar phrase, motto, or synopsis of a story. You might write about misstatements. I once wrote a series of devotionals about words children heard incorrectly. One of them was the child who prayed to Howard (as in "Howard be thy name").
Another time I wrote a series of seven on the idea of choosing the kind of day we wanted to experience—and started by saying the idea came after the fourth person in an hour had said, "Have a good day."
Devotionals aren't merely clever stories with a Bible verse tacked on. Instead, you write to integrate stories that make the Bible more alive. Choose topics with which readers identify—often small, everyday happenings. One series I wrote was on the small things that irritated me. In the series of seven, I used each reading to show that small irritants pointed to deeper issues.
Above all, make the message relevant. You can write about an experience in World War II or an event from the War of 1812, but they must have meaning for today. A friend wrote a series centered on The Count of Monte Cristo. He showed that although revenge inflames us to action, only love satisfies.
Inspirational writing takes the mundane and shows heavenly meanings.