The signing marks the beginning of a five-year push in churches, mosques, synagogues and temples across the world to send the message that slavery is forbidden by God.

But perhaps most extraordinary was that this historic meeting, which overcame traditions of religious suspicion and enmity amid the manicured gardens and marble columns of Vatican City, was largely inspired by the determination of a Perth schoolgirl - now a confident young woman.

Grace Forrest, 21, sat proudly in the front row at the 16th-century Casina Pio IV villa as the pledge was signed in front of her, after an hour of speeches in five languages.

It is six years since the daughter of mining billionaire Andrew "Twiggy" Forrest first witnessed the evil of child slavery, she told Fairfax after the event.

She was 15 when she arrived on a school trip to Nepal, having chosen to work in a refuge home for children whohad been rescued from sex slavery.

"That experience changed the course of my life," she said. "To see something and walk away, that's unacceptable to me. Seeing the physical, emotional and mental trauma of children as young as three who have been rescued from slavery is unbelievable. And to watch children who have had not just their innocence stolen from them, but also their childhood, left to live with trauma for the rest of their lives ... I came back from this experience and I really wanted to do something."

But when she returned to Nepal two years later with her family, it was to an even bigger shock: none of the children were still at the orphanage she had been working at.

"I started asking questions – where are the records, I was told 'there are no records'. I showed pictures, 'we have never seen these children'. The driver who took us out there was saying 'this is a bad place, this is a very bad place'.

"To realise at the age of 17 that these children ... had been abused again, and had fallen through the cracks because no-one was watching was a pretty eye-opening experience and made it very raw and real for our family. From there we felt a responsibility."

There was a Forrest family meeting in the kitchen back at home in Australia in 2011. They realised that opposing slavery would pit them against corrupt and powerful groups.

"We were told we were about to take on some incredible enemies," Ms Forrest said. "My mum understandably was saying 'is Grace going to be a target?' … I said 'if we don't do this now, this is what I'm going to do anyway later'. My mother said 'OK, we stand with you'."

Grace with her father took the first steps in creating the Walk Free Foundation, an anti-slavery organisation that last year joined forces with the Vatican to launch the Global Freedom Network, which was behind Tuesday's pledge.

"There's immense pride," Ms Forrest said. "I feel like a puppet for hundreds of thousands of girls who are voiceless – if I can stand for them, that is what I'm here to do."

Pope Francis said at Tuesday's gathering that "each and every one of our creeds" was united in declaring that modern slavery in every form was a crime against humanity.

"Modern slavery continues to be an atrocious scourge present on a large scale throughout the world," he said. "It is hidden behind closed doors, in homes, in the streets, in cars, in factories, in fields, [and] in fishing boats."

He called on people of faith, governments and businesses to support and join the movement against slavery.

Hindu spiritual leader Mata Amritanandamayi told of her personal encounters with "thousands upon thousands of examples of human trafficking", including a woman dying from AIDS who had been forced, at age nine, to work in a brothel.

However, she warned that eliminating child labour, a form of slavery, could lead to more suffering if it was not accompanied by support for those children's families.

Buddhist Zen master Thich Nhat Hanh, in a message read by his eldest monastic student, said compassion would "help traffickers to wake up and stop what they are doing" – but "the roots of modern slavery run deep" and would require a "new global ethics".

Jewish leader Rabbi Dr Abraham Skorka said liberation from slavery was an ancient part of the Jewish story, and one of his faith's central messages was "the courage and profound meaning inherent in being a free person".

In a message read to the meeting, Mohamed Ahmed El-Tayeb, Grand Imam of al-Azhar, said slavery was "deemed reprehensible in Islamic law" which "considers the emancipation of slaves as one of the greatest acts of worship which please God".

Iraqi Grand Ayatollah Mohammad Taqi al-Modarresi said every human being had the right to dignity and freedom.

He hoped that the Global Freedom Network would grow into a significant international body like UNESCO, allowing faiths to "work together to save humanity from violence and discrimination".

Anglican archbishop Justin Welby thanked Andrew Forrest for playing a "key role" in making the day happen. He was "dazzled" by the achievement, he said: "I haven't worked out its full potential, but I know the potential is huge."

Mr Forrest, who made an emotional speech thanking all the participants, later told Fairfax that he felt an enormous sense of relief having overcome all the obstacles to bring all the religious leaders together.

"There's a sense of history here and we are simply the facilitators," he said. "[The religious leaders] needed to show forgiveness and love to come here. Any one of them could have derailed this entire meeting and there were some serious bumps along the way but none that derailed the train.

"I think it's time, it's dawning on the world how vicious and how severe the global slavery problem is. It was a common and wonderful campaign which all the faith leaders could agree upon.

"We were really able to develop that sense of common identity, of protectors of the least on Earth that led to the miracle of this day, Shia and Sunni, Islam and Christianity and the other faiths. This is the start of a journey, the first step."

He would not comment on the next step, but Fairfax understands that next year the GFN will start a program in faith communities across the world to push the anti-slavery message, including prayer days.

Australia's ambassador to the Vatican, John McCarthy, is on the global council of the GFN and said the Australian government had supported Walk Free by facilitating access to the Vatican and helping with contacts with other faith leaders.

"Today was a matter of enormous joy and of great satisfaction," he said. "[It was] an important milestone for interfaith co-operation over one of the greatest evils of the modern day.

"This is a call to action."

Joint Religious Leaders Declaration Against Modern Slavery

"We, the undersigned, are gathered here today for a historic initiative to inspire spiritual and practical action by all global faiths and people of goodwill everywhere to eradicate modern slavery across the world by 2020 and for all time.

In the eyes of God*, each human being is a free person, whether girl, boy, woman or man, and is destined to exist for the good of all in equality and fraternity. Modern slavery, in terms of human trafficking, forced labour and prostitution, organ trafficking, and any relationship that fails to respect the fundamental conviction that all people are equal and have the same freedom and dignity, is a crime against humanity.

We pledge ourselves here today to do all in our power, within our faith communities and beyond, to work together for the freedom of all those who are enslaved and trafficked so that their future may be restored. Today we have the opportunity, awareness, wisdom, innovation and technology to achieve this human and moral imperative."

*The Grand Imam of al-Azhar uses the word "religions"

Signed by:

• Catholic: Pope Francis

• Hindu: Her Holiness Mata Amritanandamayi (Amma)

• Buddhist: Zen master Thich Nhat Hanh (Thay) (represented by Venerable Bhikkhuni Thich Nu Chan Khong)

• Buddhist: The Most Venerable Datuk K. Sri Dhammaratana, Chief High Priest of Malaysia

• Jewish: Rabbi Dr Abraham Skorka

• Jewish: Chief Rabbi David Rosen, KSG, CBE

• Orthodox: His All-Holiness Ecumenical Patriarch Bartholomew (represented by His Eminence Metropolitan Emmanuel of France)

• Muslim: Mohamed Ahmed El-Tayeb, Grand Imam of al-Azhar (represented by Dr Abbas Abdalla Abbas Soliman, Undersecretary of State of al-Azhar al-Sharif)

• Muslim: Grand Ayatollah Mohammad Taqi al-Modarresi

• Muslim: Grand Ayatollah Sheikh Basheer Hussain al-Najafi (represented by Sheikh Naziyah Razzaq Jaafar, Special adviser of grand ayatollah)

• Muslim: Sheikh Omar Abboud

• Anglican: Most Reverend and Right Honourable Justin Welby, Archbishop of Canterbury