Last Friday night, as I was heading home along London’s Carnaby Street after a drink with a friend, a boy in his late teens cycled towards me.

He seemed to recognise me as he stopped, smiled at first, then said: ‘Heil Hitler.’ Then he pedalled off.

I nearly didn’t mention the episode publicly because I didn’t – and don’t – feel like a victim.

I was not threatened. I was not frightened. Frankly, after years spent prosecuting and defending allegations of racially aggravated crime as a barrister, discussing extremism publicly and trying to hold people to account regardless of any ideology, I understand that ugly people sometimes say ugly things.

But there was one detail I could not shake: I was not shocked. That should concern all of us far more than one idiotic child on a bicycle. Somewhere, somehow, this boy had absorbed the idea that ‘Heil Hitler’ was now an acceptable thing to shout at a Jew in central London. Jews are becoming fair game again.

Not that Britain is full of Nazis. Of course it isn’t. The danger is subtler than that: the normalisation of anti-Semitism through distortion, denial and whataboutery. The gradual creation of a culture in which hostility towards Jews is endlessly explained away as politically understandable.

Again and again, the same message emerges. ‘What about Palestine?’ ‘What about Netanyahu?’ ‘Surely Jews should call this out.’

Jewish suffering comes with conditions attached. That should alarm anyone who cares about British values, because British values are fairness, restraint, the rule of law and the protection of minorities.

Historically, the Left understood where anti-Semitism leads. These were movements that marched against Fascism at Cable Street – when thousands of Londoners gathered to stop Oswald Mosley’s British Union of Fascists marching through the East End. Trade unionists and liberals once recognised anti-Semitism instinctively as a poison that threatened the whole democratic order.

Now, many seem morally paralysed. I say this with sadness because much of the work I do in Holocaust education – teaching students and appearing in documentaries – is based on a very simple belief that prejudice against one minority eventually threatens every minority.

That is why the refusal of huge numbers of so-called ‘progressives’ to confront anti-Semitism is becoming so morally disfiguring.

Yes, of course there is real, ugly and dangerous anti-Semitism on the Right of British politics, too. But what makes this moment so culturally disorientating is seeing people who loudly claim the language of tolerance and human rights now unable, or unwilling, to confront the same poison in their own ranks.

Yesterday, the Daily Mail reported that about 30 Green Party members and election candidates are under investigation over allegations of anti-Semitism and racist abuse.

Then came Green Party grandee Caroline Lucas condemning anti-Semitic comments from within her own party. One might imagine such a statement would be uncontroversial. Instead, much of the online response accused her of being a traitor, insufficiently committed to the cause. Hanging over all of it is the absurd suggestion that because Zack Polanski is Jewish, the party itself is somehow inoculated against anti-Semitism. As though the mere presence of a Jewish party leader magically cleanses prejudice around him.

A terrifying coalition has now emerged within the ranks of ‘be-kind’ Left-wingers. These movements, which claim to be passionately committed to LGBT and women’s rights and liberal freedoms, are now marching alongside rabid activists who openly despise those very values. What happened to the Left that once instinctively defended gay people, Jews and liberal democracy?

Keir Starmer says ‘never again’ to Jew-hatred. Most decent people agree with him. But slogans are not enough.

Not while Jewish students are being warned about visibly identifying themselves on university campuses. Not while synagogues and schools resemble fortified barracks. Not while Jewish men are stabbed in terrorist attacks in Golders Green. Not while Jews are told that hostility towards them is somehow understandable, ‘contextual’ or deserved. Recently, I attended a reunion of the 45 Aid Society in London – a Jewish charity founded by Holocaust survivors. Security had been tightened (that sentence alone should stop us in our tracks).

At my table was a teenage Jewish girl from a comprehensive school in east London. She quietly explained that she no longer wears her Star of David publicly because boys at school regularly greet her with ‘Heil Hitler’. She is called simply ‘the Jew’ there.

Almost every Jewish person I know now belongs to at least one WhatsApp group in which a once-unimaginable question is being discussed seriously: where would we go if things became worse? What other minority community in modern Britain would be mocked, dismissed or accused of hysteria for admitting to fears like that?

My grandfather himself arrived here after the Holocaust as one of the Windermere Boys: children liberated from German concentration camps and brought to Britain after the war. This country gave him not merely refuge, but a profound love of his home itself and of the democratic inheritance that had saved him.

He believed passionately in British democracy and liberty because he had experienced life without them. People who have touched the face of tyranny tend to value freedom differently from those who merely inherit it.

Which is why the silence, dithering and moral evasiveness from large parts of the activist class, and the wider political establishment, now feels so shameful.

In polling stations across Britain, people cast votes thinking, reasonably enough, about tax, schools and bills. But elections are also about the kind of country we wish to become. About the moral atmosphere we are prepared to tolerate. And history teaches that bystanding is never neutral.

After all, in November 1938, the conductor Karl Bohm led a performance of Bruckner’s Fifth Symphony at the Vienna Konzerthaus. Critics described it afterwards as a glorious evening and the audience applauded magnificently. Then they stepped outside – into streets where synagogues were burning during Kristallnacht.

That is the lesson history actually teaches. The slide into barbarism does not begin with death camps. It begins when civilised people learn to rationalise hatred. When prejudice becomes conditional. When minorities are told, subtly or otherwise, that perhaps they somehow brought the hostility they suffer upon themselves. And when good people decide to say nothing.

The boy who shouted at me last week was not born hating Jews. Adults taught him.

The question now is whether enough people in this country still possess the courage and the British decency to stop being bystanders. To speak up when Jews are abused, to stop excusing hatred because it arrives wrapped in fashionable politics. To show that this country still knows the difference between solidarity and silence.