A powerful 17th-century portrait of the Christian martyr St Agatha has gone on display at the Georgian mansion and country estate where it was once a star of the art collection.

The painting, by the devout Catholic Florentine artist Carlo Dolci, was bought by the National Trust last year to return it to its “home” of Osterley House in west London.

It is a survivor of a jaw-dropping art collection assembled by the banker Sir Robert Child, most of which was catastrophically destroyed in a fire in 1949.

The Dolci was sold in the 1930s. It was bought at auction by the trust last year for £248,750 with money that included £85,000 from the Art Fund.

John Chu, assistant curator of pictures and sculptures at the National Trust, said the painting going on display was a powerful moment on a number of levels. It represents a “homecoming” and as a status symbol illustrates the unexplored story of the Child banking family, whose fortune created the estate.

But the painting itself means so much to many people. It tells the story of Agatha, who, as punishment for her Christianity during Roman times, was raped and had her breasts cut off. Her tears are ones of pain and suffering, but also of joy and salvation as a vision of St Peter appears to her and heals her wounds.

St Agatha is the patron saint of victims and survivors of sexual assault, as well as breast cancer patients.

“The mix of intense beauty and horror in the picture is there in order to evoke something which is above beauty and horror,” said Chu. “It is about the divine, salvation, the everafter.”

The Dolci has gone on display in a specially constructed walk-in ‘vault’, which could just as easily be interpreted as a chapel.

It is part of an exhibition opening on Monday, telling a less familiar story for visitors to Osterley, most of whom go there because of its amazing interiors – the work of architect and designer Robert Adam, who from 1761 remodelled the house into the ‘palace of palaces’.

Others might go there because it was Wayne Manor in Christopher Nolan’s 2012 Batman film The Dark Knight Rises.

The exhibition of art, furniture and porcelain tells the story of the Child banking family, which counted William and Mary, Nell Gwyn and Sir Isaac Newton among its clients.

Osterley once had one of the nation’s most important country house art collections, with works by artists including Rubens, Van Dyck and Claude.

The National Trust was given the house and estate after the second world war, but not the paintings. There was, however, an intention to donate them at some point, obliterated by the 1949 fire.

“Osterley will never be the great picture house that it once was,” said Chu. “Which is why any pictures which escaped that fire … it is important to acquire them. They are beautiful fragments of what has been lost.”

• Treasures of Osterley – Rise of a Banking Family is at Osterley House, London, 4 November-23 February