There's much to admire about Jeremy Corbyn. Even his detractors must grudgingly admit that this is a man who believes what he says and says what he believes - unlike, as is commonly held, the majority of modern politicians.

But serious questions persist about the company he keeps.

Take, for example, Paul Eisen, the self-professed Holocaust denier . In a blog post, Mr Eisen said that he had known Mr Corbyn for more than 15 years; that Mr Corbyn has donated money to his anti-Israel group; and that the Islington North MP has attended "every single" one of his annual events.

(When the story emerged this week, Mr Corbyn's spokesperson said that his "office" had had no dealings with Mr Eisen, but could not say the same about the MP himself.)

READ: The key questions Jeremy Corbyn must answer

Or take Stephen Sizer, the vicar who was barred from social media for sharing antisemitic conspiracy theories, and has given a speech at a Holocaust-denial conference in Iran.

When he came under fire for his alleged antisemitism, Mr Corbyn wrote a letter of support claiming he was "under attack" because he "dared to speak out against Zionism".

Similarly, consider James Thring, the conspiracy theorist who speaks of "Jewish elders" pulling the strings of the financial system; last year he gave a speech in Parliament at an event hosted by Mr Corbyn (the MP maintains he didn't know that Mr Thring was going to speak, but the fact that he was able to do so with impunity can speaks volumes about the nature of the event).

Last but not least, consider the hate preacher Sheikh Raed Salah, who served two years in prison for raising funds for Hamas and working for Iranian intelligence. Mr Corbyn was filmed praising him as an "honoured citizen" and inviting him to "tea on the terrace" at the House of Commons.

As shocking as all of this is, none of it was ever kept secret. Which leads us to an overwhelming question: Why does nobody seem to care? The answer is as obvious as it is uncomfortable.

Opposing Israel has become the dominant totemic cause for left-wingers in recent decades, magnetising the energy that was previously expended on apartheid South Africa, Irish republicanism and other charismatic causes of yesteryear. It has also become a convenient symbol of the "Western imperialism" that so irritates the far-left.

Many of those campaigning against the Jewish state may be acting out of deeply held political and social convictions (even if others would argue passionately against them).

But the bloated, disproportionate focus on Israel has taken on a life of its own, and become a lightning rod for antisemitism.

This has led to an absurd state of affairs in which the likes of Sheikh Raed Salah - whose views on Jews, homosexuality, women's rights and democracy are entirely at odds with secular liberalism - can find allies in Parliament.

This unholy alliance can only allow antisemitism to hide in plain sight. Indeed, these days it can be difficult to identify which parts of the anti-Israel movement are motivated by racism, and which parts are not. Which makes people suspicious of Corbyn.

Ultimately, as the old song has it, you can tell a man who boozes by the company he chooses.

It would be wrong to demand that Mr Corbyn abandons his genuine support for the Palestinians, even if it may be deluded. But when it comes to his unsavoury bedfellows, he would be wise to get up and slowly walk away.

Jake Wallis Simons is a novelist and journalist