Vincent Doyle was 28 years old when he discovered his beloved godfather, a Catholic priest who had died years before, was actually his biological dad.

Standing in the old stone parish house in Ireland where his father served in the ministry and where he spent many happy times as a boy, Vincent recalls being ecstatic at the news.

"[It] was the antidote to the worst day of my life, that I had lost him — because I got him back," he tells Foreign Correspondent.

"There was a huge kindness to him. He was a father in everything but the word."

The stunning revelation set Vincent on a journey which would see him uncover the scale of a global cohort of children fathered by priests.

Vincent Doyle (right) pictured with his "godfather", who he later learned was actually his biological father. ( Supplied )

Many, he would discover, were closely guarding their secret and grappling with shame while yearning to know their dads.

What Vincent did would help countless others like him and put the Catholic Church on notice that these "hidden children" were determined to be heard.

The hidden ones

Initially, Vincent Doyle was surprised there was so little information online to help him deal with the truth about his parentage.

But he was certain he wasn't alone.

Vincent approached the Dublin Archbishop, Diarmuid Martin, and received seed funding for a digital service to offer assistance to other children of priests.

"I thought, 'well, let's put up a website where people can start downloading information, like an information pack for free. Maybe it will help people'," he said.

"I thought I could just move on with my life and that would be it."

He never imagined what it would lead to.

Vincent Doyle founded Coping International as a support network for children of Catholic priests. ( Foreign Correspondent: Ron Ekkel )

Within weeks of setting up the charity Coping International, Vincent was astonished to see how many people from around the world were reaching out for information.

To date, over 80,000 visitors from 175 countries have used the website, often arriving after Googling phrases like "my father's a Catholic priest", "alimony for priests' kids" or "help I'm pregnant and the father is a Catholic priest".

Vincent runs the Coping International website from his home office in rural Ireland.

Its primary service is sourcing free professional mental health care for the children of priests and their parents, anywhere in the world.

As word of the service spread, so did the number of people making contact through the site.

Based on the number of hits received on his website and other information, Vincent estimates there could be at least 10,000 children of priests globally.

"Priests are breaking their celibacy vow in every country in the world. As long as you have priests, you will have children of Catholic priests."

Vincent's goal is to support others who have made a similar discovery as him.

But he also understands the warm, loving relationship he had with his own father was the exception to the rule.

The more common experience for children of priests is rejection and enforced secrecy, along with all the hurt and shame such treatment can cause.

'I was the object of his shame'

Shame and stigma are feelings British-born Sarah Thomas endured for years.

She found out her biological father was a priest when she was 12 years old. Today, she's 41 and is still struggling with the impact of this discovery.

"It's very lonely being a priest's child, it's very isolating and it's very disempowering," said Sarah.

Sarah's mother fell pregnant while she was studying at university — her partner was training to be a priest. When she told him the news, he was furious.

British girl Sarah Thomas learned her dad was a priest when she was 12 years old. ( Supplied )

With support from an older priest, the men made it clear to Sarah's mother she would only receive financial support if she stayed silent.

It meant she had to keep the truth from her daughter.

When Sarah was a teenager, she urged her mother to tell her everything. At 14, Sarah pushed for a meeting with her father, who was by then an ordained priest.

"I was convinced that if he met me, he would be thrilled and … sad that he'd missed out on those years," she said.

But it was not a happy encounter.

"Seeing him come into the room, very cold, I suddenly had this shame. I felt full of … self-loathing that I was the object of his shame and that there wasn't anything I could do."

As a young woman, Sarah realised she would never be able to have a relationship with her father, because "his priority was to protect his reputation and to continue the secrecy".

From that moment, her life spiralled out of control. As she struggled to come to terms with her family history, Sarah's schoolwork suffered, she drank too much and partied too hard.

Sarah Thomas's meeting with her biological father was not a happy encounter. ( Foreign Correspondent: Ron Ekkel )

"I think it nearly killed me … I internalised the hurt to the point where I'd stopped looking after myself."

Today, she believes there were two key moments in her life which helped her to move on.

At 20, she almost died in an accident. During the months she spent recovering in hospital, Sarah felt she had been given a second chance.

Sarah's second life-changing moment was discovering Coping International's online community and meeting other children of priests.

Finally, she wasn't alone.

After a lifetime of hurt and isolation, Sarah said she and others like her deserve to be heard.

"I think priests' children as a group want to be acknowledged. They want to be on the map. They exist. They're not collateral damage."

Sarah is now doing a PhD on issues surrounding children of priests and she takes an active role in reaching out to others.

Which is how she met South Australian woman Linda Lawless.

'I now have to carry the secret'

Linda Lawless, from Mount Gambier, was eight years old when her mother admitted her "dad" wasn't her real father. But she refused to say anything more.

"She used to have a look of fear on her face and you just knew … not to go there," said Linda.

Linda didn't learn who her biological father was until she was in her 50s. That he was a Catholic priest made the news even harder to bear.

Linda Lawless learned as a child not to ask her mother about who her father was. ( Supplied )

It was only after both her parents died that Linda started digging.

She learned that her mother, who was in her early 20s and unmarried when she became pregnant, almost had Linda adopted.

Then at the last moment, Linda's mother changed her mind. Linda's birth certificate didn't register the name of the father.

Linda wanted more information. She took a DNA test and discovered a strong match to a family called Kelly. Her aunt's response to that news helped Linda fill in the missing pieces.

Years before, Linda's mother admitted to her sister who the child's father was. Father Joseph Kelly was a Catholic priest in Victoria and a close family friend.

Linda Lawless started digging for information on her biological father after her parents died. ( Foreign Correspondent )

"When I found out who my father was, the stigma became bigger again and I remember thinking, I can't tell anybody. I now have to carry the secret."

But once she encountered the online communities on Coping International, Linda felt she had discovered a lifeline.

"To be able to communicate with other children of priests is very important, especially in the journey of recovery of the trauma that each one of us all goes through."

The fear driving secrecy

For Vincent Doyle, the welfare of the children of priests is vital and he's pushing church authorities to be more transparent in dealing with the issue.

After he lobbied the Congregation of Irish Bishops, they issued guidelines on what the church must do when a Catholic priest fathers a child.

The guidelines say the welfare of the child must come first, and do not compel the priest to leave his post.

Vincent is now calling on the Vatican to do the same. But so far, it has refused to make its guidelines public, instead referring to them as "internal technical notes".

Foreign Correspondent went to the Vatican, which refused to reveal internal guidelines on what the church must do when a Catholic priest fathers a child. ( Foreign Correspondent: Ron Ekkel )

Foreign Correspondent went to Rome to ask Vatican spokesman Andrea Tornielli why the guidelines haven't been made public. He said only church officials and the children of priests are permitted to see them.

Vincent strongly objects to the principle that priests who father children should leave the priesthood.

"No person, man or woman, should be forced from their job for becoming a parent … 'Congratulations, you're a father. You're fired!' This is what's driving the secrecy," he said.

"These men, our fathers, my father. All these men are terrified."

Although he acknowledged the effort the Church has made to improve the way it handles these matters, Vincent said it has not gone far enough.

"They have done something good. Is there a lot more room for improvement? You bet your bottom dollar there's a lot more room for improvement," said Vincent.

"A Pope has never spoken openly about children of priests."

In the meantime, many children of priests have found solace in meeting others who have shared their experiences.

Visitors from around the world have visited the Coping International website for information on priests fathering children. ( Supplied )

And they all agree on what they want — the right to speak openly about who they are and to be acknowledged by the Catholic Church.

"I'm not doing the lies and the secrecy anymore," said Linda.

"I'm not carrying that shame. I have to step past it."

Watch Foreign Correspondent 'Secrets and Lies' tonight at 8pm on ABC TV and iview