Easter? The very word gives me a migraine.” Not my view, but that of an old family friend who couldn’t shake the folk memory of Easter as pogrom season, a time of anti-Jewish attacks as Christians resurrected the libel that it was the Jews, rather than the Romans, who killed Jesus. But this weekend is also Passover, when Jews retell the story that defines them as a people, sitting around a Seder table and recalling through words, song and, crucially, food their exodus from slavery in Egypt.

The Easter/Passover combination means that at this time every year Jews are reminded of two core facts about themselves. The first is that they are raised, from the start, to remember that their place is with the oppressed and against injustice because, were it not for the exodus, they would still be slaves today. The second is that, from the start, they have been hated.

Both of those messages feel timely this weekend, as Jews reflect on the way a movement that they long saw as their natural home – on the left, fighting oppression and injustice – has been rocked by the question of anti-Jewish hatred. In the last week, the Passover and Easter messages have collided painfully.

Rather than rehash the evidence again, perhaps it might be useful to pick our way through some of the responses that have greeted this latest eruption, starting with the notion that antisemitism is being “weaponised” against the Labour leadership.

It’s quite true that the issue has been picked up by those on the right with no love for Labour, or for Jews for that matter. It’s hard to take seriously the outrage of the Mail or Telegraph when both have reached for the antisemitic dog whistle in the recent past, attacking Ralph Miliband or George Soros using the familiar old codes. You can be particularly disgusted by Leave.EU, which tweeted an image suggesting Labour was turning on Jews in a bid to win over British Muslims. It was factually wrong, sought to pit one minority against another, and was instantly condemned by mainstream Jewish organisations.

So yes, you can make a strong case that plenty are acting in bad faith, trying to use this issue as a stick to beat Labour – but if you do that, you need to exempt Jews themselves from that charge. As one who knows this community well, I can tell you: what’s motivating those Jews protesting about antisemitism in Labour is fear of antisemitism, no more and no less. It’s wrong to suggest their true purpose is thwarting the Corbyn project, as if the Jews who demonstrated in Westminster on Monday are pretending to be outraged by anti-Jewish racism when their real motive is stopping the renationalisation of the railways.

This needs to be stressed because what lies beneath such a view is a notion that is itself antisemitic: that Jews do not act sincerely, but always with an ulterior motive or hidden agenda.

What about the view, tweeted by the former minister Chris Mullin, that Jewish leaders were “ganging up on Corbyn” because of “criticism of Israel”? Besides offering a neat example of the ulterior motive cliche – with the added hint that the elected representatives of an ethnic minority numbering 300,000 are more powerful than the leader of a mass membership political party on the brink of government – it also falls apart on the facts. For the key prompts in the current controversy – a mural showing hooked-nosed Jews counting money, and Holocaust denial posts from a would-be Labour councillor – have nothing whatsoever to do with Israel.

Indeed, what has Jews anxious now is the resurfacing of old-school antisemitism, unmoored to the conflict in the Middle East. They are hearing again all the old tunes – Rothschilds, conspiracy, money – replayed on a leftist keyboard.

Nor will it do to say that a political party will always reflect wider society, that for as long as there are antisemites in the UK there will be antisemites in Labour. For one thing, the left exists to change society, not simply to reflect its existing defects: it’s right to expect better of Labour than of other parties. But it also betrays a deep misunderstanding of this phenomenon.

For the antisemites exposed within Labour – and monitoring groups reckon they have documented racist posts by at least 1,000 party members, forwarding formal complaints about many of them – have not wandered into the wrong party by mistake. They’re not BNP-types who misread the sign on the door. On the contrary, their racism is a warped deformation of their leftism.

Remember, antisemitism differs from other racisms in its belief that Jews are the secret masters of the universe, pulling the strings that shape world events – and always for the sake of evil. (Witness the former Labour mayor of Blackburn who suggested Israel was behind the Sandy Hook school massacre.) Once you swallow that canard and see the Jews as the wielders of clandestine, malign power, why, then it becomes your duty as a good leftist to fight the Jews. This is why August Bebel called antisemitism “the socialism of fools” more than a century ago – and the Tower Hamlets mural illustrates that doctrine perfectly.

None of this is new. These ideas have been around on the far left, and on the edges of Labour, for many decades. But they were marginal before because the far left was marginal. Now the far left is in charge. Should anyone else, besides Britain’s tiny Jewish community, care about any of this? One Morning Star columnist wrote this week that if you’re poor and homeless, then “antisemitism accusations don’t figure much in your daily list of getting by”. The implication is that it’s impossible to fight both poverty and antisemitism, and that a bit of the latter is a price worth paying for defeat of the former.

Do we need to spell out why this is a mistake? Perhaps we do. Maybe it’s hard to see antisemitism as a threat equal to other racisms. The implications seem less obvious for Jews than they might for Asian or black Britons: Jews are rarely the target of calls to crack down on immigration, for example. But a change in the political climate could have a concrete effect on Jewish life in this country.

Less tangibly, it’s the cast of mind, the way of thinking, that antisemitism represents that we should fear. Conspiracy theory, fake news, demonisation of an unpopular group: what happens to our politics if all these become the norm? This is why Jews have often functioned as a canary in the coalmine: when a society turns on its Jews, it is usually a sign of wider ill health.

Put another way, hasn’t history shown us that racism never stays confined to mere “pockets”? Once the virus is inside, it does not rest until it has infected the entire body.

• Jonathan Freedland is a Guardian columnist