London. A man gets up one morning and leaves home with a knife in his pocket and violence in his heart. He walks down to his local Tube station and randomly stabs people in the name of a cause he believes in and a religion he practises. He is not Muslim. He is Jewish.

At least, he is Jewish for the purpose of this hypothetical version of last weekend's events at Leytonstone, for which Muhyadin Mire is scheduled to appear at the Old Bailey charged with attempted murder. His acts have been described by police and the prosecution as terrorism. Witnesses say he knifed the throat of a 56-year-old man while shouting "This is for Syria".

But back to the hypothetical version. For reasons that will become clear, let us swap Muhyadin for, say, Moshe, who, as he knifes his victims, shouts, "This is for Israel". And just as happened with Muhyadin, Moshe is tasered, arrested and, while he is being held on the ground an onlooker shouts "You ain't no Jew, bruv," to which, in this now increasingly fictional version of last weekend's events, the Jew on the ground replies, "Yes I am".

Now, on this, I'm with the Jewish terrorist. Because if Moshe was raised as a Jew or converted to Judaism or identifies as a Jew, then he's Jewish. And just because there are a lot of Jews -possibly including the man who shouted "You ain't no Jew bruv" - who deny his Jewishness because they abhor his actions and understandably don't want to be associated with them, it doesn't mean he's not Jewish.

Turning from the hypothetical back to the actual, the man at Leytonstone Tube whose voice was recorded shouting "You ain't no Muslim, bruv" in the aftermath of the attack has made bigger headlines than the knife-wielding assailant he was shouting at. His comment has trended on Twitter, David Cameron has applauded it and the Times has written an admiring leader on the remark.

Now, no one in their right mind would oppose the apparent sentiment behind the witness's comment. It could only have been said by someone who is appalled by an extremist minority's attempt to appropriate a religion from a peaceful majority.

For most Muslims, the way Islamic State claim their faith must feel like the worst form of theft. But as well-meant as the witness's comment undoubtedly is, it was wrong. He is a Muslim. Just as, if he were Jewish, someone saying "You ain't no Jew, bruv" would not make him any less so.

The comment is the latest wrongheaded attempt by some Muslims and non-Muslims to distance Islamist terrorism from what they call "real" Islam or "true" Muslims. This culminated in the wake of the Paris attacks with the Home Secretary Theresa May declaring that the terrorists "have nothing to do with Islam".

Following that and other attacks, some Muslim commentators have said that ordinary Muslims should not feel obliged to apologise for crimes they had no part in. But that is to somewhat disingenuously miss the point. Apology would be symbolic, it's true. But unless the peaceful majority take ownership and responsibility for acts carried out in their name by a violent minority then who else is going to tackle it?

When, last July, Jewish terrorists, to use the Israeli Defence Forces term, fire-bombed a Palestinian home on the West Bank in which an 18-month-old baby boy was burned to death - an attack that later resulted in the baby's parents also dying - it would have been absurd to claim that the people who did it were not "real" Jews and had "nothing to do with Judaism". Would those who distance Islamist extremists from "ordinary" Muslims also suggest that "ordinary" Jews and Israelis should not feel implicated by the actions of Jewish terrorists? Probably not. But if they did, they would be wrong.

Because if Jews don't take responsibility for Jewish terrorism, who will? Some Jews have. While describing his hospital visit to the victims last August, Israeli President Reuven Rivlin spoke of his shame, while Israeli opposition leader Isaac Herzog urged Israeli Prime Minister Benjamin Netanyahu to order Shin Bet, the Israeli security services, to "tackle Jewish terrorism like [it does] Muslim terrorism."

Rivlin is right. The "ordinary" majority should feel shame for the crimes committed in their name by a violent minority. This applies as much to "ordinary" Muslims as it does to "ordinary" Jews, just as "ordinary" Hindus should be the first to confront the rise of militant Hindu nationalism in India, which a couple of months ago led to a mob armed with bricks dragging a Muslim man from his bed and killing him because he was rumoured to have eaten beef.

The man who shouted, "You ain't no Muslim, bruv" has done our fearful world a service, especially if he himself is also a Muslim. He has brought people together with a simple expression of disgust.

But how much more potent it would be if "ordinary" Muslims responded to it with a chorus of "Yes he is".