The Global Freedom Network, the brainchild of billionaire Australian magnate Andrew Forrest, will pressure governments and businesses to free millions of men, women and children held in bondage around the world by 2020.
"Today, the economic exploitation of our fellow human beings causes almost 30 million people to be enslaved, more than at any time in human history," Forrest said at the launch.
The Vatican, the Anglican Communion and Cairo's Islamic al-Azhar University are leading the initiative, which "brings together faith communities of almost three billion people -- nearly half of the world's population -- and will invite all faiths to join its leadership," he added.
Representatives of all three met at Vatican to sign up to the initiative, which Forrest said would bring together "millions of churches and mosques to gather in one great army to fight the war against slavery."
The network will call on the world's 50 biggest corporations to root out exploitation of workers in their supply chains and ask the Group of 20 most advanced nations to set up a global fund to finance programmes tackling slavery.
Mahmoud Azab, on behalf of al-Azhar, told AFP the network is a "strong appeal against the egotism of rich countries, against materialistic societies which must stop all forms of aggression against the human being".
"Modern slavery and human trafficking are one of the greatest scandals and a tragedy of our age, which touches every part of the world in some way," said David John Moxon, who represented Welby.
In 2013, the Walk Free Foundation published a Global Slavery Index, covering all forms of bondage from people trafficking to children forced to wed, which estimated that 29.8 million people live in slavery worldwide.