Here's an interview with Enger, and here's a quote:
Jeremiah Land is a deeply religious man who performs Christ-like miracles and apparently carries on conversations with God. Are you a religious person? How did your faith influence this book?
LE: Well, I'm a Christian—a decision I made when I was Reuben's age, 11 years old. I don't know how you write a book without your faith showing up in it. If I were a practicing Buddhist, a pantheist, or an atheist, you would expect to see that evident in my fiction. Your faith has everything to do, I think, with the way you see the world. And since my world-view is a Christian one, that's how my work is going to read. That said, the book is not an attempt to evangelize. It's really the story of a boy coming to grips with the faith he has been raised in and seeing it played out in terms of loyalty and sacrifice. I think those are things that matter, whether you're Christian or whether you're some other faith. If somebody writes a book making sure that no article of his faith, even if it's only a questioning of his faith, gets into the work, what kind of book is that? I don't think it's a novel. Maybe it's a math text.
I've been asked whether Peace is a Christian novel, and that label troubles me. Does it mean a novel written by a Christian? In that case, of course it is. But I think right now, and maybe it's been this way for a long time, there's kind of a milk toast connotation. Christian novel is taken to mean something tamed beyond all interest or something overtly evangelical, whereas what I think of as Christian novels are those that point out man's need for redemption. Crime and Punishment, Robinson Crusoe, Les Miserables, that wonderful one by Graham Greene, The Power and the Glory, all those books declare that man is incapable of saving himself, of delivering his own redemption. Yet we don't call those Christian novels, we call them classics.