There has been a church on the site of St Stephen's since 1304, and today the place of worship, set back off Corn Street in Bristol city centre, offers an enchanting combination of 15th-century stonework and 19th-century fabric.

But Bristol artist Graeme Mortimer Evelyn is currently working on an addition to this ancient space, which will insert a vibrant 21st-century aesthetic into the ever-evolving building.

Hidden in a little workshop, tucked away at the back of the nave, Graeme – one of the founding members of the Jamaica Street artists' collective – is adding the finishing touches to his masterpiece.

Using bright primary colours, in thick enamel paint, Graeme is bringing to life the figures of St Stephen and St Sophia, which he has painstakingly carved in that most modern of materials – MDF.

Later this month, the four new panels will be incorporated into the church's traditional, ornate stone altarpiece, which has been covered by a curtain almost since it was created in the 1870s.

Despite Victorian architect Charles Hansom's months of work, the resplendent "Agnus Dei" – lamb of God – which figures as the centrepiece of the screen (or "reredos" in church-speak) was quickly covered-up by the incumbent vicar, who felt the iconography was "too Catholic".

When Canon Tim Higgins arrived at the church more than 130 years later, he peeled back the curtain.

"It seemed extraordinary that this wonderful altarpiece had been covered up for so long," he says. "The four niches featured Victorian figures representing the Ten Commandments, painted on tin panels. But they had rusted and needed replacing."

Tim's mind turned to artist Graeme Mortimer Evelyn, whose work on a new set of Stations of the Cross figures at Gloucester Cathedral had already caught his attention.

Tim knew the project would need to be approached with the utmost sensitivity.

History has given St Stephen's the dark, unenviable boast of being the harbour church for the city in the 18th century, when the slave trade flourished in the port.

It was here that slave traders took communion, though it was also here that early abolitionist Rev Josiah Tucker spoke out against the grim "African trade".

"I quickly realised the new reredos could act as a focal point to our programme of reconciliation," Tim says. With financial backing from the Heritage Lottery Fund, the Bristol Legacy Commission, and Bristol City Council, the reredos project was able to take shape.

When he arrived at the church for the first time, Graeme – who is himself of Jamaican descent – realised he had quite a challenge on his hands.

"I didn't want to produce a mock-Victorian reredos," he says. "So my challenge was to create a contemporary piece that blended well with the rest of the Victorian altarpiece and the medieval setting.

"I wanted the piece to serve Tim's mission of reconciliation, but I also wanted it to be timeless enough so it also worked as a focus of worshippers' meditation both now and 300 years from now. Although raised a Christian, I'm now a practising Buddhist. So I spent a lot of time researching the biblical context before I did anything.

"I wanted to suggest reconciliation without being too overtly specific about the slave trade. So I decided to include St Stephen himself – one of the Christian church's first martyrs, who was remarkably under- represented in the iconography of the building, even though the church is dedicated to him.

"And it was suggested that St Sophia be the second figure – as she is the patron saint of wisdom, and links East and West."

The two saints appear in vibrant colours on the outermost panels – colours that tie in with the hues of the stained glass running through the enormous east window over the altar.

"The inner two panels feature two faces of adoration," Graeme explains. "They could be any of us, and like Cain and Abel they are individuals finding their differences reconciled in looking towards God – who is represented here, at the centre, by the lamb of God stone carving."

● The new reredos will be officially unveiled at a special ceremony on Friday, January 21, from 6pm, which will include an address by Kwame-Kwei Armah, the radical Christian playwright, actor and broadcaster. All are welcome.