It's a documentary that Windsorite director Matt Gallagher has been aspiring to create for about 15 years — and now, his film Prey about sexual abuse by Catholic priests will premiere at Hot Docs, Canada's largest documentary film festival later this month.

The film focuses on one perpetrator in particular, Father William Hodgson "Hod" Marshall, a retired priest and teacher, who several years ago pleaded guilty to sexually abusing 16 boys and one girl at schools in Toronto, Sudbury and Windsor.

Featured in the film is Windsorite Patrick McMahon who, as a boy, fell victim to Marshall.

McMahon has been using his voice to speak out and protest in an effort to hold those within the church accountable.

"It's something I feel passionately about....I will continue to speak out until people who cover this up are brought to justice," he said.

Watch the film trailer below.

He stressed he hopes the documentary will help make people aware these are not just crimes of the past.

"There are priests today who are still doing this. There are priests being investigated now. There are enablers covering this up," he said.

"We all together have an obligation to make that stop."

McMahon has been represented by Rob Talach, a lawyer based in London, Ont. — known as "the priest hunter" — and he too is a prominent figure in the film.

Gallagher reached out to Talach in an effort to focus the documentary on a case that was unfolding in the present, and so they were able to identify one of Talach's clients who was taking his case to trial, and that's where the documentary begins.

'White knight work'

Having tackled more than 400 cases in his career so far, Talach says after 17 years of representing individuals who have been abused by Roman Catholic clergy, he's become a "six foot tall callus" emotionally.

Patrick McMahon hopes the documentary will help raise awareness about crimes of this nature. (Katerina Georgieva/CBC)

"There's no way to completely turn this off and not have it affect you," he said.

"But this is the type of work that ... I always say is 'white knight work'. It's true fight for justice. These are society's most vulnerable. They were children at the time. Their life paths have really been altered by this. And this is an opportunity to, in a sense, fix a past wrong and change for the future."

Moving forward, he said, it's important to pressure institutions to put systems in place and cultures in place that won't allow this to ever happen again. Tallach suggested that one solution would be to remove celibacy for priests and allow women to be priests.

For Gallagher, it was challenging to take on a documentary dealing with stories of abused children.

"Emotionally, it was very difficult," he said.

"The most chilling aspect for these stories for me was that after keeping this secret for so long, these survivors, when they finally started to tell people, often they weren't even believed, and that's the hard part for me."

A facilitator at SNAP who had survived abuse talks about her thoughts on the documentary. 2:58

McMahon acknowledged that not everyone who has been abused is able to come forward with their story the way he has been able to, but he encourages people to reach out to someone or reach out to an organization like the Survivors Network of those Abused by Priests, and get the support they need.

"I strongly recommend, find at least one person you can confide in and get the help that you need."

Church responds

CBC reached out to the Roman Catholic Diocese of London for its reaction:

Father William Hodgson Marshall was transferred to different communities, but remained a teacher. (Border City Pictures)

Communications officer Nelson Couto said, by email, that he hasn't had a chance to see the documentary. But he said that he hopes the documentary will increase awareness and help survivors move forward in their lives.

"Sexual predators come from all walks of life: mothers and fathers, teachers, doctors, people of all colours and creeds," Couto said.

"Obviously celibacy isn't the issue, which is sickeningly evident to anyone who reads the news on a regular basis. It's also obvious, in other faith traditions where clergy are married and of both genders, that marital status and gender are irrelevant," he said.

He added that the Diocese of London holds the conviction that the focus must be to seek healing for survivors, and stop abuse from happening again.

"We need to be on guard everywhere, and at all times, and those are the principles that the Diocese of London has used since the 1980s in our goal to prevent abuse from happening."