I overheard a conversation in late September of 2001. I was sitting on a Manhattan subway on my way to give a talk on medieval Muslim-Christian relations at New York University. A construction worker on his way to the clean-up efforts at the World Trade Centre was talking to a friend, and they were debating the causes of 9/11.

“The Jews turned Manhattan into a symbol of capitalist greed,” said one, “that’s why the Arabs hate us.” His friend agreed, adding: “and also because they killed Christ.”

I was immediately struck, and struck hard, by two things. The first was the familiarity of the explanations. Very similar things might have been said, for example, by a 15th-century Christian trying to explain a famine or a plague. And the second, almost the opposite, was the adaptability of the explanations, the ease with which they could be put to new kinds of work, such as the work of explaining to two New Yorkers why their city had just been attacked at the beginning of the 21st century.

I decided then and there that I needed to explore both sides of this phenomenon, both the continuity, and the change. Why is it that thinking about Jews and Judaism has done so much work for so many different peoples and cultures seeking to explain their world? And how has this thinking been transformed over time, in order make sense of an ever-changing cosmos?

We can’t focus on only one period in history if we want to understand both how anti-Judaism endures, and how it changes. So I cast my net wide, across the centuries, reaching from the contemporary to the ancient world, and across religions (Christianity, Islam), languages, and cultures.

What I found was that, in many of these cultures — including many cultures where no Jews had lived for centuries —”Judaism” became a basic concept with which people tried to make sense of their world, and the overcoming of “Judaism” one of their basic ideals.

Why do I put “Judaism” in quotation marks here? Because this concept of “Judaism” had little to do with anything that real, living Jews might think, believe, or do (although it could certainly affect real Jews). Instead, this “Judaism” that needed to be overcome could be produced from within these cultures themselves, as happened most notably within Christianity and Islam.

You can already see this process at work in the scriptures of both religions. For example, in Galatians, when St Paul is arguing with St Peter about how gentile converts should follow Jesus, he accuses St Peter of wanting the gentiles to “Judaise” (2.14). And he explains that by “Judaising,” he means giving priority to the letter of scripture over its allegorical meaning, and to the flesh over the spirit.

When a follower of Jesus does this, says Paul, he “Judaises,” which is to say, he in some way becomes a Jew. Of course Paul and Peter were themselves both Jews, and knew something about real Judaism. But what is important to notice is that already here, in one of the earliest surviving documents from a follower of Jesus (circa 50 AD), “Judaising” doesn’t mean being a living, confessed, Jew. Instead it means anyone (in this case gentile followers of Jesus) paying excessive attention to the literal meaning of a word, or to the body, or to the “Law,” or to the material world.

In other words, Paul’s “Judaising” is an attitude to the world, not a religion. But here is the problem: is there any way to read a word without paying attention to its literal meaning? Is there any way to live in the body without paying attention to the flesh? Or to live in human society without law? If not, then everyone, no matter how Christian, is in constant danger of “Judaising.” In effect, when Paul warns Christians not “to Judaise,” he puts a potential “Jew” inside every Christian, and turns “Judaism” into a key term by which Christians can criticise themselves and each other.

Later Christians developed this potential and put it to work in new ways. We even find saints trading accusations, as when Augustine worried that Jerome’s translation of the Bible from Hebrew gave too much power to the Jews, and Jerome responded by accusing Augustine of converting Christians to “Judaism” through his literal interpretations of scripture.

It is this widespread deployment of “Judaism” as a critical concept in early Christianity and early Islam that made it such a basic concept in the cultures that inherited those traditions. Anti-Judaism in those cultures became a way of trying to understand and overcome the gap between the real and the ideal, between the ever-changing and chaotic world as we encounter it in our bodies and through our senses, and the transcendent and eternal truths we yearn for.

Over time and space, in different places and periods until more or less our own day, these ideas about Judaism have been transformed in order to make new sense of an ever-changing world.
The Reformation, the Enlightenment and the French Revolution, Idealist philosophy and Marxism, modernity and mass democracy: all of these produced, and were in part produced by, these new ways of putting anti-Judaism to work.

I call this system of thought anti-Judaism, rather than antisemitism, because the latter is a narrow sub-set of the former. Antisemitism is the particular, racialised form that Western European anti-Judaism took in the 19th century when it encountered hundreds of thousands of Jewish immigrants from Eastern Europe.
But anti-Judaism is a more basic and a more powerful system of thought, more basic, because it explains why so many people can make sense of their world in terms of the dangers of “Judaism,” even when there are no Jews in their world at all. More powerful, because it can be applied to anyone of any religion, or indeed of no religion.

In fact, in an important sense, the vast majority of anti-Judaism’s victims have not been Jews by confession. In ancient, medieval, and early modern societies (many of them with vanishingly small Jewish populations), countless Christians and Muslims were attacked as “Judaisers” by their co-religionists.

Even under the Nazis, many people were condemned as “Jews” who were not Jews by “race” or by confession. Similarly today in the Muslim world, the ideological power of anti-Zionism (which may sometimes be a form of anti-Judaism) affects many millions, of whom Jews are a tiny minority. Not only Israeli Zionists, not only Israelis, not only Jews, but also Europeans or Americans can all be resisted or attacked as Zionists or allies of Zionism.

So can Muslims, ranging from Anwar Sadat (the president of Egypt assassinated after concluding a peace treaty with Israel) to the Islamicist rebels in today’s Syria (characterised by Bashar al-Assad as lovers of Israel), to secularising protesters in Tehran.

Anti-Judaism may help us understand why these discourses are so convincing to so many, in a way that antisemitism cannot.

In short, approaching anti-Judaism as a system of thought may help us to approach the present as well as the past. Once we have a sense of just how central anti-Judaism has been to the history of our concepts and tools of thought, it should be easier for us each to interrogate our own sense of reality, and to want to cultivate a critical sensibility about those moments when we might be projecting anti-Judaism into our vision of the world.
Jew or non-Jew, we should all want to cultivate such a critical sensibility: after all, anti-Judaism has a powerful effect upon the possibilities for everyone’s existence in the world, whether Jewish or not.

David Nirenberg is professor of medieval history and social thought at the University of Chicago. His book, ‘Anti-Judaism’, is published by Head of Zeus, £25