While most art-lovers are drawn to Belgium’s rich store of Brueghels and Magrittes, there is another work that some might argue is the country’s prize artistic possession: French painter Jacques-Louis David’s depiction of the fatal stabbing of a French revolutionary in his tub. The scene happens to be one of the most glorious lies ever told.

Why is a masterpiece of such renown and historic significance hanging on the walls of Brussels’ Museum of Fine Arts instead of in the Louvre? Why is it tucked away in a quiet corridor rather than conspicuously displayed in a place where French museum-goers could admire it on their own turf?

The short answer is that the fate of the painting – like the fates of its subject and creator – were not all that glamorous.

David (1748–1825) was a brush-for-hire. He painted for aristocrats, revolutionaries, kings and emperors, continually shifting his allegiance, putting his talent at the service of the ruler of the day. As a young artist, he painted an entire soap opera of classical figures. His representations of Seneca, Socrates, Andromache and Brutus filled canvases with heavy drapery, heaving bosoms and glinting swords. He captured the likenesses of his elite Parisian contemporaries as well, including that of the famous (and ultimately beheaded) chemist Lavoisier posing next to his 18th-century laboratory equipment.

But David’s crowning achievement was yet to come. It would take the death of a pustule-ridden French revolutionary to establish him as an unsurpassed political propagandist.

The revolutionary was Jean-Paul Marat (1743–1793), hot-headed editor of the revolutionary newspaper L’Ami du peuple and the author of countless death lists for the guillotine. Marat was the most fearsome and influential figure among the Jacobins during the Reign of Terror: his prose was Robespierre’s most deadly weapon.

But Marat was a sorry sight: covered in flakes and boils from a skin infection that was said by one doctor to have originated in the ‘perianal’ region, he worked in a cold bath, scribbling away on an upturned fruit-crate. He had formed a friendship with the painter David as a brother-in-arms and a brother-in-pain.

David could empathise with the writer’s chronic discomfort since he himself suffered a facial disfigurement which made his speech laboured and difficult to comprehend. Although neither could claim to be the Revolution’s pin-up boy, between them they possessed the most incendiary pen and paintbrush of their generation.

Yet, it was a grim and vulgar act which propelled them to stardom. One sticky day in July 1793, the story’s femme fatale, the revolutionary Charlotte Corday, made her way from Caen to Paris with the mission of killing Marat. As a member of the revolution’s more moderate Girondist faction, Corday considered Marat a murderer and was prepared to sacrifice her life in order to save a thousand others. She entered his room under the pretence of bringing him a list of prospects ripe for execution. After handing him the document, she stuck a kitchen knife in his chest. He bled to death in the bath while guards bundled her to the ground with little resistance.

Completed four months later, David’s painting idealises that ugly scene. His brush cures Marat of his lesions and leaves him lolling peacefully in the tub with a small incision in his chest and a light smear of blood on Corday’s letter, which remains in his hand. Parallels with Christ’s body taken down from the cross are blatant: David depicts Marat as a martyr, serene, slit in the side, limp like the dead Saviour lying at the foot of the cross. There are no signs of suffering or decay, just a smooth arm reaching out imploringly to the spectator.

Thousands of copies of the painting were made and distributed throughout France. Towns were renamed after Marat and occasionally his bust replaced sculptures of Christ in churches. In David’s painting, which was displayed in the Louvre, the Jacobins had their icon and their hero. The Reign of Terror was in over-drive.

But in the shifting sands of 18th-century Paris, the painter’s security was never assured. Five days after Robespierre’s execution, David was denounced by association and although he escaped the guillotine, he didn’t evade prison. He was incarcerated for a few months until the next leader took office and he could offer his services again. In 1794, a year after completing the Marat painting, David had already returned to safer subjects with his Intervention of the Sabine Women, and by 1800 he was painting Napoleon in self-glorifying poses. In 1816, he was, however, finally banished from France when the royal family returned to the throne.

David dodged his enemies and eventually settled in Brussels, where he spent his final 10 years. He kept his head down, remarried his estranged wife and painted portraits and histories to differing standards. The incendiary portrait of Marat remained in his possession but it was judged too politically risqué for public display and was kept at his son’s house. Viewers had to have a police permit to see the painting.

One winter evening in 1825, David, then 77, stepped out of the theatre and was hit by a carriage. The French authorities, still fearful of the painter’s legacy, would not allow his body to be repatriated. Eventually he was buried in the cemetery at Saint-Josse-ten-Noode. His heart was later transferred to Paris’ Père Lachaise cemetery.

David’s paintings remained in the possession of his family, who sent most of them back to Paris to be auctioned. The Marat, however, was judged too treacherous to cross the border. Eventually in 1886 the family offered it to the city which had granted its creator exile in the last years of his life. Here it still hangs at a safe distance from France, a reminder of art’s former power to incite.

The portrait has since become a template for political martyrs. It is an enduring image of self-sacrifice taken up by generations of activists across the political spectrum. Gone are the memories of Marat as a mass-murderer, a delusional and diseased megalomaniac. Instead, we are left with the beatified face of a ruthless regime. David’s painting has shown dictators how to be remembered for their legendary leadership rather than for their crimes. Such was David’s success that as late as 1921 Russia renamed one of its battleships Marat.