On Easter Sunday, 1475, the body of a 2-year-old Christian boy named Simon Unferdorben was discovered near a Jewish home in the northern Italian city of Trent, then a province of the Holy Roman Empire. Immediately, authorities arrested every Jew living in the city. Even as they awaited sentencing, a mythic narrative sprang up around the dead toddler: that he was a saintly martyr, a victim of ritual murder by the Jews, who had killed him to use his blood in the baking of their Passover matzohs.

After prolonged torture, eight Jews were beheaded or burned at the stake for their alleged roles in the boy’s death. The late-medieval Christian networks of communication broadcast the myth of Simon’s martyrdom at the hands of a cabal of evil Jews across languages and countries. Simon’s tomb became a magnet for pilgrims from across Christendom, who claimed that the saintly boy had granted them miraculous healing from hernias, stabbings and other ailments. The bishop of Trent financed the writing of poems and hagiographies that both praised the martyred child and denounced the perfidy of Jews.

If, in the 1400s, anti-Semitic myths were transmitted by means of beatific poetry, legends of martyrs and claims of miraculous healing, today such lies are transmitted far more easily.

But it is a mark of how unchanged such myths are that John Earnest — the 19-year-old man who police say entered a Southern California synagogue, killed a 60-year-old woman and injured several more people last weekend — apparently claimed to have acted, in part, to avenge the death of Simon of Trent.

“You are not forgotten Simon of Trent, the horror that you and countless children have endured at the hands of the Jews will never be forgiven,” said a manifesto in Earnest’s name that was uploaded to the text repository Pastebin and posted on the anonymous message board 8chan. The dissemination of the manifesto was instantaneous, reaching thousands of readers around the world. But the hatred at its core could have been plucked directly from the 15th century.

Over the past 500 years, anti-Semitism has evolved to suit changing times, but it has never receded. The rhetorical utility of redirecting popular anger against a reviled minority has retained its power; therefore, so has the notion that Jews are a nefarious fifth column, a “synagogue of Satan” whose sole urge is parasitism and destruction. Where once Jews were accused of poisoning wells to spread bubonic plague, contemporary white-nationalist rhetoric posits instead that Jews seek the destruction of the “European” or “white” race by encouraging interracial marriage and mass immigration.

If the manifesto on 8chan is any guide, Earnest subscribed fully to this pernicious myth, which far-right ideologues call “white genocide,” and preached it as sincerely to the Web-savvy readers of his text as any medieval clergyman ginning up a pogrom by spreading the blood libel. (That the majority of American Jews tend to lean politically left, and therefore generally endorse more open immigration policies and oppose racism, provides a far-right, radicalized, conspiracy-drunk subculture all the proof it needs.)

Like the manifesto of Brenton Tarrant, whom authorities have charged with slaughtering Muslims at prayer in Christchurch, New Zealand, in March, the one apparently written by Earnest is larded with jokey references to message board culture, memes and YouTube stars. In the 8chan post with the link to the manifesto, the writer said he planned to soundtrack his atrocities with a number of “memeable” songs, including the Pokémon theme song. But beneath the superficial ironies and jokes, both documents are deadly serious — and filled with exhortations for others to incite racial conflict through slaughter.

Earnest’s alleged manifesto is a call for both homicide and suicide, a terrifyingly nihilistic screed that casts the slaughter of Jews as a noble act against an implacable foe. “I would die a thousand times over to prevent the doomed fate that the Jews have planned for my race,” it says. There is a striking casualness to the way the writer discusses murder, a video-gamey distance from the horrors he said he planned to inflict; he called slaughtering as many Jews as possible achieving a “high score.” According to Bellingcat analyst Robert Evans, the alleged killer’s 8chan thread and Facebook page were full of fellow message board members urging him on, telling him to “get the high score.”

It’s easy to look at a man wielding an assault weapon, receiving and disseminating radicalized ideas through the Internet and urging supporters to create memes lauding his actions, as a profoundly modern phenomenon. But in the end, it’s not all that different from the bishop of Trent commissioning poems to create the myth of a martyred saint and ordering Jews burned at the stake. The goals are the same: to cast Jews — each individual Jew, and Jews as a whole — as the driving force of the world’s evils and to build, on their corpses, a world cleansed of sin.

Hatred of Jews, no matter how savvy its technological promulgation, retains the crudity and the violence of a medieval mind-set. In such a worldview, a child saint can cure a painful hernia, every individual Jew is plotting the dilution of the white race through miscegenation, murder is a sacred duty, and holy war is a mere provocation or two away.

That such a mind-set proved fatally appealing to a teenager in California is frightening; that the movement is international, metastasizing from Norway to Pittsburgh to New Zealand to Poway, Calif., is indisputable. The only way to fight such sentiments is to recognize them for the ancient, cruel and enticing prejudice they represent.

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