When I helped bring the Modern Slavery Bill to parliament I thought here, surely, was a piece of legislation that the PM would want to own. Three women — Theresa May, the Home Secretary; her then special adviser Fiona Cunningham; and Philippa Stroud, Iain Duncan Smith’s special adviser — had all worked for a Bill that would give the government a chance to seize the moral high ground, restoring Britain to its historic role as leader in the abolitionist movement.

David Cameron was within touching distance of greatness. But almost at the last moment, he stumbled. Wary of alienating the business community, he balked at the idea of stipulating that quoted companies must report on how they were checking their supply chains against their use of slave labour. He thought this would introduce an unnecessary regulatory burden on businesses.

In fact, a host of big businesses from Tesco to the Co-op, from Primark to investment bankers like Rathbones, would welcome the legislation. Slavery (by which we mean not just human trafficking but also domestic servitude) is the second largest global criminal industry. Slave labourers are invisible: it is difficult for the T-shirt retailer in London to know the labour conditions of the cotton pickers in India. By not forcing companies to conduct due diligence, to declare their supply chains slave-free, Cameron has denied them the best line of defence: ‘I checked my suppliers, as the law demands.’ This leaves them open to accusations of illegal practice and — just watch — lawsuits.

The PM has missed an opportunity to make the Modern Slavery Bill a world leader by setting the best legislation possible to help destroy this evil practice, but I hope the churches won’t. They too need a great new cause to burnish their tarnished image. I know that individuals in both the Lords and the Commons, and across all political parties, are going to fight for amendments to the Bill; even Theresa May would prefer to see legislation on supply chains.

This is about convincing No. 10 Downing Street; and the churches — Anglican, Roman Catholic and Nonconformist — must join in this battle. By loudly and very visibly promoting a new abolitionist campaign, they can be seen as the champions of the tens of thousands of people in the UK alone and millions abroad who are the victims of slavery. What better way to remind secular Britain of its traditional role as a guardian of true morality?

Banishing slavery is a universally popular cause, and a nonpartisan one. This is precisely what the churches need, to disarm critics who claim they have become mouthpieces of every trendy campaign imaginable. A new anti-slavery campaign would be free of party politics. To stamp out a trade that affects 30 million people around the world is not a question of Labour vs Tory, as both sides are committed to the Bill. The question is how we make this Bill a world leader which other countries will seek to adopt.

Many of the churches’ past policies have angered the right, yet surprisingly have not completely alienated them. The churches’ Conservative-leaning members remain loyally on side. Indeed, despite their recent difficulties, the church institutions still mobilise the biggest numbers in terms of activists and signatories for petitions. In an election year, this matters: even the most mild-mannered churchmen can inspire real fear in MPs in swing seats.

The churches’ campaign ought to centre on the 50 most marginal Tory, Labour and Lib Dem seats. In many of these areas, the MPs are hanging on by the skin of their teeth. A relentless campaign from church constituents and the signing of petitions will, as Dr Johnson said about hanging, concentrate the MPs’ minds wonderfully on political survival.

If the faithful push for a supply-chain clause to the Bill as it comes up for its last Commons debate in mid-October, I think many MPs will have no choice but to listen; all the more so if parishioners and constituents decide to name and shame those politicians who refuse to come on board.

So let me say this clearly: I challenge the churches to draw inspiration from the great Christian abolitionist, William Wilberforce, and galvanise their followers to stamp out slavery. Now as then, if they choose, they can be a great lobbying force for good.

On this issue of a Modern Slavery Bill, the churches have a chance to make the world more just. Such opportunities do not present themselves that often.