It took me 40 years to become a Jew. When I was a child, I wasn’t a Jew and not only because I never went to a synagogue. My father’s family had abandoned their religion so he wasn’t Jewish. More to the point, my mother and my grandmother weren’t Jewish either, so according to orthodox Judaism’s principles of matrilineal descent, it was impossible for me to be a Jew.

All I had was the “Cohen” name. I once asked my parents why they had not changed it. After saying, quite rightly, that you should never seek to appease racists, they confessed to thinking that antisemitism was over by the 1960s. After Hitler, humanity would surely see where the world’s most insane hatred led and resolve to put it to one side.

Bertolt Brecht said: “Do not rejoice in his defeat, you men. For though the world has stood up and stopped the bastard, the bitch that bore him is in heat again.”

My parents did not believe Brecht, at least not in the 1960s. Nor did I for a while. I was and remain an atheist who knows that communalist and identity politics crush individuality. I had no wish to join a tribe, let alone a religious one.

Still there was no escaping the “Cohen”. When I first responded to the antisemitism that has spread so far from the extreme left into the mainstream that it now threatens to poison the Labour party, I am ashamed to say I considered two disgraceful replies.

I might, I thought, not stop at opposing the Israeli occupation of the West Bank, and pledging support to leftwing Israelis and Palestinians who wanted a just and peaceful settlement for both peoples, but go on to behave like a grotesque from a Howard Jacobson satire. I would reassure fanatics that their “anti-Zionism” (that is, their call for the total destruction of the world’s only Jewish state) was not remotely racist.

Fortunately for my self-respect, I never sank that low. Whenever I hear Jews announce their hatred of Israel’s very existence, I suspect that underneath their loud bombast lies a quiet plea to the Islamists and neo-Nazis who might harm them: “I’m not like the others. Don’t pick on me.”

Unfortunately, I assured anyone who asked (and some who did not) that, despite appearances to the contrary, I wasn’t Jewish. And that was as dishonourable. I sounded like a black man trying to pass as white or a German arguing with the Gestapo that there was a mistake in the paperwork.

I stopped and accepted that racism changes your perception of the world and yourself. You become what your enemies say you are. And unless I wanted to shame myself, I had to become a Jew. A rather odd Jew, no doubt: a militant atheist who had to phone a friend to ask what on earth “mazel tov” meant. But a Jew nonetheless.

As one of the finest liberal ambitions is to find the sympathy to imagine the lives of others, you should become a Jew too. Declare that you have converted to Judaism or rediscovered your Jewish “heritage” and see the reaction. It’s not just that, if you are middle class and fortunate, you might experience racism for the first time, which in itself would be a “learning experience” worth having. You might also learn the essential lesson that antisemitism is not about Jews. Like rape, it’s about power.

Whether the antisemitic conspiracy theory is deployed by German Nazis or Arab dictators, French anti-Dreyfusards or Saudi clerics, the argument is always the same. Democracy, an independent judiciary, equal human rights, freedom of speech and publication – all these “supposed” freedoms – are nothing but swindles that hide the machinations of the secret Jewish rulers of the world.

Describe the fantasy the Tsarist and Nazi empires developed that bluntly and it is impossible to understand how the Labour party is in danger of becoming as tainted as Ukip by the racists it attracts.

But consider how many leftwing activists, institutions or academics would agree with a politer version.

Western governments are the main source of the ills of the world. The “Israel lobby” controls western foreign policy. Israel itself is the “root cause” of all the terrors of the Middle East, from the Iraq war to Islamic State. Polite racism turns the Jews, once again, into demons with the supernatural power to manipulate and destroy nations. Or as the Swedish foreign minister, Margot Wallström, who sees herself as a feminist rather than a racial conspiracist, explained recently, Islamist attacks in Paris were the fault of Israeli occupiers in the West Bank.

Or consider the otherwise bizarre indulgence of ultra-right religious extremists by people who otherwise describe themselves as liberals and leftists. The belief that Jews fuel radical Islam allows them to overlook superstition and the tyrannical denial of equal rights. They’re against Israel and that’s all that matters.

I could describe at vitriolic length how disgusted leftwing Jewish friends are that Labour members chose Jeremy Corbyn, despite his support for an Anglican cleric who linked to extremist sites that blamed Jews for 9/11, and his defence of an Islamist who recycled the libel that Jews dined on the blood of Christian children from the bottom of a medieval dung heap.

But even if a chastened Labour expels this or that antisemite or disciplines the Jew-baiters at the Oxford University Labour club, I do not see how its leaders can challenge the conspiratorial world-view they shared for decades. They would be renouncing everything they once believed in.

As someone who warned in the 00s about the growing darkness on the left, I am pessimistic about the chances of change. If you keep shouting “fire” and the fire brigade never comes, you tend to assume the house will burn to the ground. But perhaps familiarity breeds contempt and I am not the best judge.

If Labour MPs and members want the party to break with a past that has led to leftists allying with religious reactionaries who deny universal human rights and hate every value the centre-left professes to hold, they will have to learn to treat all racisms equally.

They will need to make a brief acquaintance with European history and understand that the left has no guaranteed immunity from fascistic ideology. They will have to see antisemitism for what it is and understand why it always leads to despotism and despair. Like me, in short, and if only briefly, they will have to become Jews themselves.

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