Yes, it's Christian-themed, but it doesn't often feel like it. The most preachy book is the first one, and honestly it's not that bad. I'd just call it, I don't know, "moral fantasy" or something. A lot of the characters are a little too nice for my taste, a little too honorable, a little too compassionate, etc. I enjoy a cast of characters that has more variety. But I didn't really dislike any of them even in a cast of thirty-something people, so Overstreet must know what he's doing.
This book does start slowly, though. Nothing really interesting happens for the first quarter of the book, and even then it picks up at a very gradual pace. Some things don't make any sense at all, and there are plot-holes galore. I might honestly have stopped reading a couple times, but I'd read the third book so I knew there was action in here somewhere.
Fortunately, by the end I liked it so much that I didn't care how slow it had been. It's not a book that I'm going to re-read over and over and over again, but it's charming. The writing is good, the characters are (except for being too noble) realistic, and the world-building is very prettily done. It's a short, sweet fantasy without much bloodshed until the end, but there's nothing too graphic even then, and if you like that kind of thing, this book is for you.