When we consider the way that Christians have been at the forefront of combating the scourge of slavery, the name that quite rightly springs to mind is William Wilberforce. During the latter part of the eighteenth century he led the parliamentary campaign against the British slave trade, fighting a 20-year battle until his persistence and courage finally paid off with the passing of the Slave Trade Act of 1807.

In years to come another name may be held in equally high esteem; that of Gary Haugen. Following the end of South Africa's apartheid in the 1980s, Haugen, an American human rights lawyer, served on the executive committee of Archbishop Desmond Tutu's National Initiative for Reconciliation. In the 1990s he was appointed by the United Nation's Centre for Human Rights to oversee its genocide investigation in Rwanda. With his team of lawyers, he was personally involved in gathering eye-witness testimonies and physical evidence from nearly 100 mass grave and massacre sites across the country.

This experience had a profound effect on Haugen and not long afterwards he founded the International Justice Mission (IJM), basing its vision on Isaiah 1:17: 'Learn to do good; seek justice, rescue the oppressed, defend the orphan, plead for the widow.' IJM is now based in 18 countries; taking on trafficking and slavery, working to fix broken judicial systems and addressing the violence, crime and corruption that blights the lives of so many. It is vital, brave and often dangerous work.

Slavery may be technically illegal in every country in the world, but there are still an estimated 35.8 million people who, to every intent and purpose, are living as slaves; possessed, exploited or controlled in such a way as to significantly deprive them of their individual liberty. This is more than three times the entire number of Africans who were shipped across the Atlantic as a result of the slave trade. Far, far more people are living in situations where they cannot rely on the rule of law to protect them from. This number, which the UN puts at 4 billion, must contend with the relentless forces of violence on a daily basis. This is violence that has not resulted from war or political conflict, but simply stronger neighbours harming and abusing weaker ones.

Powerlessness and violence are overarching realities for entire populations and IJM has made it its mission to break these chains of oppression through the work of lawyers, investigators, social workers and community activists – and through prayer.

In the last ten years, IJM has rescued more than 23,000 people trapped in slavery and 21 million are being protected from violence through training police forces and partnerships with those working in local justice systems. In the UK IJM has been instrumental in the formation of the Modern Slavery Bill and in Cebu, a city of 2 million in the Philippines, its work over four years in one district that was notorious for child prostitution and sex-trafficking has resulted in a 79 per cent reduction in the availability of children in the sex trade. IJM attributes this success in some of the darkest places in the world to the power of regular and committed prayer.

I first came across IJM about two years ago and was blown away by their commitment to restoring the lives of the oppressed around the world and their reliance on God's strength to do it. I watched Gary Haugen speak in front of 5,000 at this summer's New Wine conference and as he finished he received the biggest and most powerful standing ovation I have ever witnessed. There was a profound acknowledgment that God was doing incredible things through their work.

Just this weekend I attended IJM's second annual UK conference and heard Vincent, international director of investigations, tell stories of finding himself in apparently hopeless situations where God has miraculously intervened.

In 2008 Vincent had been posted to IJM's office in Calcutta in India. Child prostitution was a massive problem and the crime lords running the brothels were feared by all, including the police. Trafficking was rife, but no one had ever been convicted of abusing these girls, many of whom were between 11 and 13 years old – they were simply not worth the effort. Vincent and his team were praying that this would change and eventually through undercover work they persuaded the police to carry out a raid of a brothel where six girls were being kept. On the day of execution, however, the area police chief decided to carry out a snap inspection of the local station, delaying the raid. The fear for Vincent was that word had leaked out and this had happened to give the criminal gang time to hide the girls. He prayed with his colleagues as their concerns mounted and he told us that as they did, he had a strong sense not to worry; that this was all part of God's plan.

Later that morning the police chief left and Vincent went with the officers to the house with little hope. What they found amazed him. Instead of six girls, there were 19. A new group of girls had been trafficked into the area and had arrived at the house, which was to be used as a distribution point, just minutes earlier. The police's late arrival resulted in a much bigger success.

As Vincent and IJM continued to work in the city, they experienced further breakthroughs. In 2010 they were able to secure 38 convictions of traffickers and crime lords including the kingpin who controlled the area. Since then the number of girls being prostituted on the streets has fallen to a small fraction compared to 2008 and it has been two years since any small girls have been seen being sold for sex. Vincent now hears the girls who were rescued talking about God releasing the enslaved. He says he has learnt to trust God far more through these experiences, and that most of all prayer changes everything.

In a video message at the start of the conference Gary Haugen explained that he wasn't going to start IJM unless he could find at least 100 people who would pray for it every day. In the end he found 123. Now, each morning, the 600+ members of IJM start the working day in prayer and then stop at 11am to pray again. It is ingrained in the organisation because as Haugen has every confidence that, in the words of Charles Spurgeon, "Prayer is the slender nerve that moves the muscle of omnipotence".

Some may look at the darkness and evil in the world and ask, 'Where is God in all of this? Why does he let it happen?' In Exodus, when Moses encountered the burning bush, God told him that he had seen the suffering and misery of his people and that he was sending Moses to rescue them. God is anything but ambivalent towards the plight of the poor and vulnerable, but in his wisdom chooses people like you and I to bring his restoration into the world.

Some like Wilberforce and Haugen will be used to change the lives of millions. For most of us it will be less monumental, but until we lay ourselves before God we will never know how much we are capable of achieving in his name. And for each one of us, there is no better way to go about this than getting on our knees in prayer.

Gillan Scott regularly writes about the relationship between Christianity and society. He is deputy editor at archbishopcranmer.com. You can follow him on Twitter @gillan_scott.