Every year on 17 February, a crowd gathers in Rome’s Campo de’ Fiori to place wreaths, poems and candles at the foot of the statue that glowers towards the Vatican from beneath its friar’s cowl. The man it memorialises, the Neapolitan philosopher Giordano Bruno, was burned alive by the Inquisition on that spot in 1600, for heresies including several books in which he advocated the heliocentric cosmology of Copernicus and argued that the universe was infinite and contained multiple other worlds.

The charges were similar to those brought against Galileo 16 years later; unlike Galileo, who recanted, Bruno’s defiance has lent an air of heroism to his execution. A witness at his trial reported that, when the death sentence was pronounced, Bruno replied: “You may be more afraid to bring that sentence against me than I am to accept it.”

Though his theory of an infinite universe was only one of many heretical ideas that landed him in trouble with the Catholic church, it has gained him a reputation in recent years as the first martyr for modern science. The physicist Neil deGrasse Tyson opened his 2014 TV series Cosmos: A Spacetime Odyssey with an episode devoted to Bruno and his place in the history of scientific thought, though he left out the fact that Bruno was just as interested in ancient magic as he was in astronomy. Brian Cox also recounts Bruno’s “cinematic death” in his bestseller Human Universe, but concludes that “his contributions to science were questionable. He was more belligerent free-thinker than proto-scientist.”

It’s as an icon of courageous free expression that Bruno’s appeal endures in Italy and elsewhere in Europe. At a benefit gig in London for the Belarus Free Theatre, I heard the activist Nadya Tolokonnikova of Pussy Riot give a speech in which she hailed Bruno as a hero of dissent in the face of repression. That a young woman in Vladimir Putin’s Russia should be inspired by a 16th-century defrocked Dominican friar is testament to the way Bruno has become almost mythologised.

I’ve been writing a series of novels about him (under the name SJ Parris) for the past 10 years, but he has long fired the imagination of playwrights and novelists, including James Joyce, Oscar Wilde and Bertolt Brecht. Victor Hugo contributed to the campaign for the statue in Campo de’ Fiori in the 19th century – a subscription campaign led by students who saw in Bruno a fitting figurehead for a Rome newly liberated from papal authority, and even credited him with predicting its deliverance. The plinth inscription reads: “To Bruno, from the generation he foresaw, here, where the pyre burned.”

The rally to pay tribute to him on Sunday in the Campo de’ Fiori was organised by the National Association of Free Thinkers, but each year it attracts an array of admirers of all persuasions: atheists, anarchists, rationalists, mystics and Catholic reformers, as well as a delegation from the city authorities who lay a wreath. This official acknowledgment of his historical importance suggests a rehabilitation of sorts, but Bruno’s relationship with authority remains uneasy.

For centuries his reputation was suppressed by the Vatican, the records of his trial kept secret, and he was denied a papal pardon on the 400th anniversary of his death in 2000, on the grounds that he had deviated too far from Christian doctrine to deserve it (Galileo was formally pardoned in 1992). There may be streets, piazzas and colleges named after Bruno all over Italy today, but in many ways he remains an outsider, a symbol of resistance. Last summer, in Venice, I came across his image stencilled on the wall of an alleyway, with no explanation. A friend in Calabria recently sent me a photo of a poster taped to a public sculpture: Bruno’s picture again, with the inscription “Giordano Bruno: against old and new Inquisitors”.

Though his reputation persists, few outside academia now read Bruno’s books; the story of his life and death has a coherence and broad appeal that his philosophical writings do not. But one of his consistent themes is the development of a new philosophy of religion that would transcend the divisions between Catholic and Protestant and even solve the religious conflicts tearing Europe apart in the late 16th century. He approached a number of European rulers, including Elizabeth I, in the hope of winning support for these ideas of tolerance and unity, but never succeeded in finding a champion.

Bruno spent half his life in exile to avoid the Inquisition, seeking refuge across Europe, being forced to move on every couple of years; he understood what it meant to be displaced. As Italy’s rightwing government turns away from tolerance and openness with its anti-immigration policies, and even the beloved fictional detective Inspector Montalbano can divide public opinion with what some have called “pro-migrant propaganda”, it seems more important than ever to commemorate the life and death of a man whose courage continues to inspire those determined to make their voices heard in the face of oppression.