At the height of the Great Depression in the early 1930s, 30 million Americans tuned their dials every Sunday afternoon to listen to the words of a Catholic priest from the Detroit suburbs.

But Father Charles Coughlin had long strayed beyond children’s catechism classes.

Instead, the so-called “radio priest” served up a mix of economic populism, staunch anti-communism and conspiracy theories — ones in which, as the decade dragged on, Jews would inevitably come to figure ever-more prominently.

Although now largely a historical footnote, Coughlin, who was born 120 years ago this week, prefigured the later rise of talk radio, has drawn comparisons with Donald Trump, and offers a historical perspective on current debates around the role of media companies in policing the boundaries of free speech.

Born in Canada, Coughlin attended a seminary in Toronto before being ordained a priest in 1916. After 10 years teaching at a Catholic college, he accepted the invitation of the Bishop of Detroit to establish a new parish in Royal Oak, a working-class suburb of the city, where he went on to build his church, the Shrine of the Little Flower.

Coughlin later claimed that, a few weeks after his arrival, the Ku Klux Klan — which targeted Catholics as well as Jews and African Americans — burned a cross in the churchyard. A sympathetic Irish-Catholic businessman, who owned Detroit’s WJR radio station, allowed the priest time on air to explain Catholicism to the community.

While his first programme in October 1926 drew only five letters in response, his audience soon began to grow, with radio stations in other Midwestern cities broadcasting Coughlin’s sermons. As the economy weakened after the 1929 Great Crash, he started to range beyond his initial religious remit.

Coughlin’s Catholicism was of a distinctly conservative theological bent. But it was also infused with notions around social justice and a concern about the costs of capitalism which had entered the Vatican’s teaching in the late 19th century and been reinforced by the onset of the Depression.

Coughlin thus railed against Godless communism, the greed of Wall Street “money-changers” and the indifference of the Hoover administration to the suffering of the “common man”.

By 1930, Coughlin’s words were being broadcast by CBS across a network of 18 radio stations; in Royal Oak, a new post office had to be built to cope with the 80,000 letters a week the priest was receiving. When CBS later decided to pull the plug on him, Coughlin assembled a network of stations willing to take his broadcasts.

It wasn’t just the content of Coughlin’s message that was appealing to many struggling Americans. He was, the historian Michael Kazin writes, “a superb entertainer who persuaded people by surprising and amusing them as much as by diagnosing the world’s ills and prescribing a cure”.

To some extent, therefore, Coughlin’s programme — “The Hour of Power” — resonated with the same intimacy and warmth as Franklin Roosevelt’s “Fireside Chats”.

But while the president — to whom he came second in a poll of the US’ most popular and powerful men — sought to appeal to the better angels of Americans’ nature, Coughlin came increasingly to deal in fear, divisiveness and hatred.

Coughlin’s early relationship with Roosevelt was close. Initially, Coughlin was a cheerleader for the New Deal — labelling it “Christ’s Deal” — but, over time, disillusionment with FDR set in, in part a reflection of his belief that the president and his circle of left-wing advisers had failed to treat him with the respect he deserved. Coughlin was enraged, too, at the Roosevelt administration’s decision to recognise the “atheistic, godless” government of the Soviet Union.

In late 1934, Coughlin launched the National Union for Social Justice, which campaigned for an eclectic mix of economic populism anti-Communism and isolationism. The NUSJ, which had over one million paying members, also began publishing a weekly newspaper, Social Justice, to propagate Coughlin’s agenda.

Two years later, Coughlin’s break with Roosevelt — who he labelled “the great liar and betrayer” and charged with being “anti-God” — was complete and his embrace of the far-right gathered pace. While his Union party was crushed in FDR’s landslide re-election, Coughlin’s taste for politics remained unsated.

The “radio priest’s” antisemitism had always been thinly veiled, with “international bankers” and “modern” capitalists a frequent object of attack. Indeed, in 1930 he attacked “modern Shylocks” who have “grown fat and wealthy, praised and deified, because they have perpetuated the ancient crime of usury under the modern racket of statesmanship”.

But, after the failure of the Union party, the mask slipped completely.

In 1938, Coughlin used the pages of Social Justice to assail “Jewish” financiers, singling out for special opprobrium Bernard Baruch, a wealthy Jewish businessman and public servant who was close to Roosevelt. The newspaper then began to serialise a version of The Protocols of the Elders of Zion. “We are not attributing them to the Jews,” Coughlin disingenuously suggested. “We are simply insisting on their factuality, be they plagiarised or not plagiarised, be they satires — or not satires”.

Coughlin’s justification echoed his equally mendacious claims that he had no quarrel with “righteous”, religious Jews, only those whose communist and atheist beliefs were attempting to “overturn a civilisation”.

As his biographer, Donald Warren, shows, Coughlin was also heavily involved in the organisation of, and served as an inspiration for, the Christian Front, an antisemitic militia, which harassed Jews and organised “Buy Christian” rallies.

Finally, however, Coughlin overstepped the mark. In the aftermath of Kristallnacht he took to the air and asked: “Why is there persecution in Germany today?” His answer: “Jewish persecution only followed after Christians first were persecuted.”

In New York, WMCA radio immediately sought to counter Coughlin’s lies. “Unfortunately, Father Coughlin has uttered many misstatements of fact,” an announcer warned listeners. The station, which accused the priest of having previously agreed to delete passages “calculated to incite religious and racial strife in America”, then pulled Coughlin’s next broadcast. Other stations began to follow suit. The hierarchy of the Catholic Church, which had been increasingly concerned at Coughlin’s rhetoric, made clear its opposition, while the Catholic Layman’s League labelled the priest’s antisemitism “cowardly and shameless”.

Naturally, in Germany, Coughlin’s claims of censorship found a sympathetic ear. “America is not allowed to hear the truth,” roared the Nazi press. “Jewish organisations camouflaged as American … have conducted such a campaign … that the radio station company has proceeded to muzzle the well-loved Father Coughlin”. The radio priest, reported the New York Times, had become the “hero of Nazi Germany”.

Unsurprisingly, Coughlin allied himself to the isolationist America First movement and opposed US entry into the Second World War — the result, he claimed of a conspiracy between the Jews, Britain and Roosevelt — even after Pearl Habour.

The US government adopted a watchful eye. In 1940, 18 members of the Christian Front were rounded up in New York by the FBI and accused of plotting to overthrow the government. In September 1941, Coughlin’s request for a passport was refused on the basis that he was “a reported pro-Nazi”.

The final blows came the following year: the FBI raided Coughlin’s church, while the attorney general ordered a grand jury probe and accused Social Justice of aiding the enemy. The newspaper was effectively shut down after being banned from using the mail.

The Bishop of Detroit, who had adopted a far less tolerant stance towards the priest than his predecessor, ordered Coughlin to stop his broadcasts.

Silenced, Coughlin retreated to his work as a parish priest. There was, however, to be no later repentance. Shortly after his retirement in 1966, he confessed to an interviewer that he “couldn’t honestly take back much of what I said and did in the old days when people still listened to me”.