The Painful True Story Behind MercyMe Singer Bart Millard's Hit Faith-Based Movie

When Christian rocker Bart Millard was first approached with the idea of turning his life story into a movie, he never imagined it would go on to become a box office hit.

Based off of his popular song of the same name, I Can Only Imagine tells the true story of Millard’s complicated relationship with his abusive father, played in the film by Dennis Quaid. The low-budget movie has made over $40 million since coming out in mid-March and continues to do strong business.

Millard’s song, which he performs with his band MercyMe, became a cross-over success nearly 20 years ago, and is still the bestselling Christian single of all time. While the singer often speaks about the song’s backstory at his concerts, the thought of sharing the intimate details of his difficult childhood on the big screen was nerve-racking at fist.

“I remember being nervous more than anything, because even though I had said yes to the movie, the idea of digging up stuff I’ve been trying to bury most of my life and then putting it up on a big screen — I wasn’t sure I was ready,” he tells PEOPLE.

Fortunately, it took nearly 8 years for the movie to develop, giving Millard, 45, enough time to psychologically prepare himself. Seeing a grief counselor after a death in the family also helped him come to terms with his childhood trauma. “If this had come out 5 years ago I don’t know if I would have been able to go through it,” he said. “I’m in a great place now but I never thought in a million years it would actually happen.”

Millard’s life changed forever at the age of 3 after his parents got divorced. When his mom remarried and relocated a few years later, his family decided it would be best if he and his brother stayed with their dad, Arthur.

“I don’t think anyone realized how abusive he was going to become,” Millard says. “I ended up living in fear for most of my childhood.”

While his father didn’t drink or have substance abuse issues, he did have a short temper and grew accustomed to taking out his frustrations on his son. “If he got embarrassed or cut off in traffic or whatever, he would take a swing at me,” Millard recalls. “I was like his punching bag.”

His life underwent another dramatic change when, during his freshman year of high school, his dad was diagnosed with cancer. His father’s illness eventually brought them closer together, and helped Millard see first hand how dramatically his dad had changed after finding religion.

“I got a front row seat to see this guy go from being a monster to falling desperately in love with Jesus,” Millard says. “By the time he passed away when I was a freshman in college, not only was he my best friend, he was like the Godliest man I’d ever known. And it’s literally changed the trajectory of my life.”

His dad’s transformation helped him discover his own faith. “I’d always been into music, but now I’m into ministry because I’d never seen a person change like that before and I’ve rarely seen it since. I guess I grew up thinking that if the Gospel could change that guy, it could change anybody. There was no denying it.”

For years while his dad went through treatment, Millard became like his nurse. The two would spend hours talking every night while Millard administered his medicine. “That’s when things really changed, because it went from us really not having much contact to talking for two or three hours every night until we fell asleep,” he says.

“The only time I ever got mad at God was not during the abusive, but when he died,” Millard adds. “I finally got the dad I always wanted and then he left. At 18, 19 years old, I was really upset and had to work through that.”

At his father’s funeral, Millard’s grandmother made an off-hand comment. “She said, ‘I can only imagine what your dad’s seeing now,’ and I became obsessed with that phrase,” he says. “As a 19-year-old, it was easier to think of my dad being in a better place.”

For years, Millard would write the phrase “I can only imagine” on anything he could get his hands on. “It was more O.C.D. than religious.”

When it came time to record MercyMe’s first album, Millard sat down with his notebook to start writing lyrics, but couldn’t find a page in any of his journals that wasn’t covered with his grandmother’s phrase. Eventually, he came to see it as a sign.

WATHC: “I Can Only Imagine (The Movie Session)” from the Newly Released Album I Can Only Imagine – The Very Best Of MercyMe.

“So I wrote the song, which was incredibly personal and special to me, but I had no idea the impact it was going to have,” he says. “I didn’t care what other people would think about it when I wrote it, it was almost like therapy to me.”

“I Can Only Imagine” was an instant hit on faith-based radio stations, but it wasn’t until MercyMe had started promoting their second album that the song became a surprise cross-over success on Top 40 stations across the country.

Asked why he believes the song has resonated with so many people, Millard says he thinks its universal message has helped. “There’s no agenda, there’s nothing dogmatic about it,” he says. “I wasn’t trying to shove anything down anybody’s throat. I think everyone, especially after the passing of a loved one, starts to hope that maybe there will be something better on the other side.”

The same, he thinks, is true of the theatrical version. “This comes down to being a father-son story that anyone can related with,” Millard says. “Whether you have an abusive father or the most amazing dad ever, every son is trying to find their way out from under the shadow of their dad. Maybe it’s as simple as that.”

Millard now lives outside of Nashville with his childhood sweetheart, and wife of 20 years, Shannon, and their five children: Sam, 16, Gracie, 13, Charlie, 11, Sophie, 9, and Miles, 6.

For the singer, showing the film to his children was a no-brainer: “I think this redemption story is a great message to pass on to my kids.”

I Can Only Imagine is in theaters now.