His faith was shaken after a terrible tragedy. And he is left wondering how a good God could allow such a nightmare to happen.
The book captured the hearts of millions in 2007, but it also sparked controversy and criticism. Some of those of those observations are resurfacing ahead of the film's release.
Award-Winning Actors Tapped for 'The Shack'
The Shack's author and the film's stars, Octavia Spencer and Sam Worthington, allowed CBN News on the set in Vancouver for an exclusive behind-the-scenes look at the production.
Oscar winner Octavia Spencer plays the role of God in the film, affectionately called Papa.
In between takes, Spencer told CBN News, "There is a beautiful journey that you have to take, that I realize now when you take on the role of God. It just shows you how human you are."
"Basically the breakthrough for me is realizing the father is 'The Father.' So, I approached it as a parent to a son," Spencer added.
Sam Worthington plays the grief-stricken dad, Mack.
A Universal Theme: Dealing with Grief and Loss
On the surface, Mack initially seems to have it all together, despite the things he has gone through. But his paralyzing trouble is revealed as the layers get peeled back.
Worthington told CBN News, "I like kind of taking on characters where they have these challenges within themselves, and then the challenges kind of ripple out."
"I've done a lot of movies that are quest movies, and this – in a way – is like that," he added. "It is a quest to discover what your faith is – and is what you believe in your heart still going to help you through the hard times."
As the book's title suggests, the setting for Paul Young's novel is a lone shack in the woods. It's a story of love and loss he initially wrote as a Christmas gift to his children.
Describing his arrival at the book, Young said, "It came from 50 years of trying to find good responses to difficult questions."
"It took me 50 years to actually become a child and then I was able to take 50 years of history and give a gift for Christmas to my kids," Young added.
From Self-Published Book to 22 Million Sold
His book went from a self-published gift to more than 22 million copies sold. And now, there is a major motion picture. It's a wonderful reality Young never really thought about.
"I made 15 copies at Office Depot and it did everything I ever wanted it to do," said Young.
"This has all been a monumental surprise. I think the grace and the kindness and the sense of humor of God."
Critics Call The Shack "Profoundly Unbiblical"
With the book's success, there have also been critics.
Some have called Young's depiction of God, Jesus Christ, and the Holy Spirit "profoundly unbiblical."
In Mack's fictional journey, he sees Jesus as a Middle Eastern man, the Holy Spirit as an Asian woman and God the Father as a black woman.
Seminary professor James B. De Young wrote a book in response to Young's book and called it Burning Down the Shack.
The criticisms are not new to Young. He heard them not long after publishing the book ten years ago. And he's comfortable hearing from those who disagree with his work.
Young said, "We need to be challenged in how we think. And a lot of times, people's responses are based in they are afraid. And I know that because I lived in that world.
"So, when a person is upset, they are not coming to tell me about me. I already know about me. They are coming to tell me in the only language they know how about what matters to them, and if I am not at risk, I can step inside that space.
"Maybe ask a good question. Maybe stay silent because it is about the dignity of the other and the journey that the other is in."
Worthington shared, "God is essentially love. He is coming at you in a form of love, so whatever helps embrace and give you that love, I think is a great idea."