When the abolitionist Thomas Clarkson gave a sermon in 1787 at Manchester Cathedral – during the city’s first mass meeting against the transatlantic trade in enslaved Africans – he saw a “great crowd of black people standing round the pulpit”.

However, little is known about Black Mancunians in the Georgian era, which makes one recently rediscovered entry in parish records at Manchester Cathedral particularly significant.

The handwritten entry offers a glimpse into the life of an enslaved African teenager who lived in the city in the 18th century, when it is estimated there were up to 20,000 Black people living in England.

Dating back to 26 December 1798, the entry says: “Indiana Mundi, aged 14. A negro girl from Congo on the coast of Africa, disposed of to Mr Paton at St Kitts & transferred from him to Arch.d Paton MD baptised this day.”

It is now expected that Indiana – and others enslaved in Manchester – will finally be honoured with a memorial at the cathedral, supported by Heritage Lottery funding. It will be unveiled on Clarkson Day, the cathedral’s annual 28 October event to confront the legacies of slavery.

Though the existence of Indiana’s unusually detailed baptism notice had been noted in earlier research, cathedral research officer Cathy Hirst recently rediscovered the original entry by chance while working through 18th-century ledgers.

Other records reveal that Archibald Paton, the man who brought Indiana to Manchester, was a Liverpool doctor who had married Sarah Burton at the cathedral just a year earlier, in November 1797.

Indiana is thought to have been a servant in the Patons’ household at a time when Black servants were a status symbol. “Exotic” names were also fashionable – Mundi, meaning “of the world” in Latin, is likely to have been chosen by the Patons.

Malik Al Nasir, a Cambridge University academic and author of Searching For My Slave Roots, explained that British people returning from Britain’s colonies brought enslaved people with them to work as house servants, footmen, farm workers or pages. Girls were “prized”, but were vulnerable to sexual exploitation.

While we know little of Indiana’s experience, “a baptism would indicate somebody’s formed an attachment and just wanted to bring them into their family”, Al Nasir added.

At the time of Indiana’s arrival, enslaved people from west and central Africa were being transported via the treacherous Middle Passage to British colonies such as St Kitts, which had about 70 sugar plantations by the late 18th century.

Baptism during enslavement was of political as well as spiritual significance for Black people. Baptism was actively discouraged throughout British colonies, Al Nasir said. Plantation owners feared that Christian teaching – particularly stories such as Moses leading the Israelites out of bondage – would encourage literacy and resistance.

There was also a widespread belief that baptism conferred legal freedom. As Al Nasir explained: “The argument was that you can’t baptise a thing, you can only baptise a person – and because he’s a person, you cannot treat him as property.”

This argument proved pivotal to the abolitionist cause. In 1771, in London – 20 years before Indiana was baptised in Manchester – an enslaved Black man named James Somerset was baptised, with three abolitionist godparents, before refusing to work any longer for his “master”, Charles Stewart. It was a significant moment in Black British history.

On Stewart’s orders, Somerset was kidnapped to be shipped to Jamaica. But the judge in the resulting court case – Somerset v Stewart – ruled that no master had the right to detain an enslaved person by force for the purpose of transporting and selling them abroad.

Somerset was a free man but it was a narrow ruling. The judge, Lord Mansfield, whose own niece Dido Belle was of mixed heritage, did not want to upset the merchant classes who profited from enslavement.

Nonetheless, the case exposed the fact that no law permitted enslavement on English soil, with the judge declaring that slavery “is so odious, that nothing can be suffered to support it, but positive law”. The ripple effects were profound.

Benjamin Franklin, the founding US politician, had been in the gallery for the Somerset case. He returned to America and reported the direction of legal thinking in London. According to Al Nasir, this contributed to the growing conviction among American colonists that as a British colony they would eventually be compelled to emancipate enslaved people – and that this prospect became one of the drivers behind the American war of independence.

In England, enslaved people in London ran away from their masters, declaring themselves free. News spread to Manchester. Masters began shipping enslaved people out overseas to retain control, while others sought passage to England precisely to seek baptism and freedom.

Yet neither the crowd of Black Mancunians Clarkson encountered in 1787 in the cathedral, nor Indiana Mundi, baptised four years later, could be assumed to be free.

The legal status of enslaved people in England remained contested, and many who attempted to assert their freedom were recaptured and deported by masters who ignored the spirit of the Somerset ruling. Meanwhile, the transatlantic trade in enslaved African people continued.

“British ships were still going to west Africa – until 1807, officially – and bringing people not just to the Caribbean and Africa, but to Liverpool, where they were selling them in the marketplace,” Al Nasir said.

Against this backdrop, Clarkson’s 1787 Manchester visit was a seminal moment in grassroots abolitionism. He had survived an assassination attempt by supporters of the transatlantic trade in Liverpool before arriving in Manchester, where he found a receptive audience. His sermon led to one in five Mancunians – 10,500 people – signing a petition against the slave trade, which was presented to parliament.

Clarkson later wrote of his visit to the cathedral: “When I went into the church it was so full that I could scarcely get to my place … I was surprised also to find a great crowd of black people standing round the pulpit. There might be 40 or 50 of them.”

Despite Manchester’s central role in an international cotton trade built on enslaved African workers, Clarkson’s signature in the cathedral’s “book of strange preachers” and Indiana’s baptism record are among only a handful of visible links in the cathedral building.

Others include a memorial to Rev Richard Assheton, a cathedral warden who inherited 244 enslaved workers and a Jamaican plantation from his uncle in 1732, and a memorial near the south entrance to Dauntessy Hulme, a cathedral benefactor who signed a petition opposing abolition in 1806.

“As an institution we have to deal with this history – we can’t just keep celebrating the fact that we were important to the abolitionist movement,” Hirst said.

Parish records offer further glimpses into Black lives in the 17th, 18th and 19th centuries: burials, baptisms and brief notations are often all that survives.

On 20 May 1757, “Philip a Negro from Mr John Mosse” was buried at Manchester Cathedral, while Eliza Alburn of Manchester, aged 22, “a brown girl from Upper Germany”, was buried at the cathedral on 26 August 1831.

Elsewhere in Manchester city centre, “Immy and Fanny, two West Indian girls, one about 15, the other about 13 years of age, natural children of Mr Campbell of Scotland”, were baptised at Cross Street Chapel in 1771, while “Frances Williams … a Black Woman” was baptised at St Mary Parsonage in 1767.