I apologise in advance for a long post.

It is always slightly telling how quickly people reach for a single culprit when antisemitism comes up.
Spend five minutes in a comment thread and someone will have decided it was all Christianity, or all Islam, or all the West, or all the East. The conclusion tends to arrive first, and the history gets dragged in afterwards to back it up.
History, awkwardly, does not play along.
Long before Christianity enters the picture, Jews are already an awkward fit in the Greek and Roman world. Not because anyone had yet dreamt up elaborate conspiracies, but because they did not behave as expected. They would not join in with civic religion. They would not hedge their bets by adding a few local gods. They stuck to their own rules.
That mattered because religion was not a private hobby. It was part of public life and loyalty. Refusing to participate was not just eccentric, it looked like opting out of the shared system. Less some grand stand, more the sort of quiet non-compliance that makes everything around it a bit harder to run.
It is not a theory of societal corrosion. It is closer to the irritation you get when one component in a standard assembly insists on its own thread pattern. The system expects everything to line up. One part refuses. Things start binding. After a while, people stop questioning the system and start blaming the part.
Then Christianity arrives and changes the nature of the complaint. The Roman version was basically practical: they do not join in. Christianity turns it into something moral. Jews are not just different, they are wrong about something central. In many strands, they are cast as having rejected the Messiah and, more seriously, as bearing responsibility for the crucifixion.
Alongside that comes a more structured idea, even if nobody at the time would have put a tidy label on it. Christianity presents itself as the fulfilment of Judaism, the thing that replaces it. If that is true, then Judaism is not simply different, it is outdated and stubbornly so. That is a much more durable basis for hostility.
Once Christianity becomes dominant in Europe, these ideas stop being occasional grumbles and become part of the furniture. They are taught, repeated, and reinforced over centuries. Ghettos, expulsions, and increasingly odd accusations follow. The hostility is no longer situational. It is just there, in the background, doing its thing.
Then, just as religion begins to loosen its grip, the whole thing is rebuilt on a different foundation. The 19th century does not get rid of antisemitism, it updates it. Out goes theology, in comes race. Now it is not what Jews believe that matters, it is what they are supposed to be. Conversion no longer solves anything. You cannot convert out of a category that has been defined as biological.
That shift matters more than anything that came before. It makes antisemitism immune to assimilation. You can change your religion, your language, your level of integration, and it makes no difference. You remain the wrong type.
You can see it plainly in the Dreyfus Affair. A modern state, a professional army, and still quite prepared to bend reality to fit the assumption that a Jewish officer must be the traitor. From there it is not a large step to the Holocaust, where the idea is taken to its logical and fairly horrifying conclusion.
After 1945, that version is thoroughly discredited, at least in public. But the underlying habit does not disappear. It adapts. Conspiracies come back with updated language. Old suspicions get dressed up a bit better, or just pointed somewhere else.
And this is the point where people assume the religious story has finished. It has not. It has just been translated. Modern antisemitism manages perfectly well without religion.
Instead of “they rejected the true faith”, you get “they are a race”, or “they control finance”, or “they pull the political strings”. The language shifts to suit the age. In the 19th century it borrowed biology. In the 20th, ideology and conspiracy. Now it often turns up dressed as geopolitics, sounding serious enough to pass without much inspection.
It is the same basic move each time. Take a complicated system, decide someone must be pulling the levers, and then point at a group you already think is a bit different. If they happen to be clustered in a few obvious roles - often the only ones they were allowed into - so much the better. You can then pretend that is proof of control rather than the result of being pushed there in the first place.
Which is why the modern argument about Zionism manages to generate so much heat and so little clarity.
Opposition to Zionism is not, in itself, antisemitic. It never has been. There have always been Jews who opposed it, alongside others who supported it. Some objected on religious grounds, some because they disliked nationalism in general, and some for a more cautious, slightly unglamorous reason.
In the late 19th and early 20th centuries, many Jews in Europe were trying to integrate into the societies they lived in. They were newly emancipated, building careers, becoming part of national life. For them, Zionism carried a risk. If Jews were arguing for a separate national homeland, it could be taken as proof that they were not really loyal citizens where they already were.
That fear was not fanciful. Accusations of divided loyalty were already circulating quite happily without any help. Zionism risked handing critics a neat exhibit to point at. So some Jews opposed it not out of hostility to other Jews, but because they thought it might make things worse.
At the same time, Zionism was not just an abstract idea being debated in European drawing rooms. It involved real migration to a place that was not empty. In the late Ottoman and early Mandate period, Jews were a small minority in Palestine, often only a few percent of the population. The arrival of increasing numbers of Jewish migrants changed the local balance, and it would have been surprising if that had not produced tension with those already living there. You do not need a conspiracy to explain that, just basic human reactions to rapid demographic change.
You can see versions of the same dynamic playing out much closer to home now. It does not take very large numbers for people to feel that change is happening to them rather than with them. If something on the scale of early Zionist migration were to happen in Britain, especially with arguments about land or sovereignty attached, it is fairly obvious how that would go. It would not stay polite for long.
History, being awkward again, managed to prove several things at once. Antisemites did indeed use Zionism as evidence of divided loyalty. At the same time, what happened in Europe persuaded many others that assimilation was not quite the protection they had hoped it would be. And in the Middle East, a local conflict took shape that had its own logic, not reducible to European arguments about Jewish identity.
If you turn to the Islamic world, the pattern is different, and less tidy. Early Islam builds in a category for Jews and Christians as “People of the Book”. They are recognised, allowed to practise, and folded into society, but on a lower rung. Not equality, but not exclusion either. More a managed imbalance that people put up with because the alternatives are not especially attractive.
For long stretches, that produces a workable arrangement. Jewish communities flourish in places like Al-Andalus and later the Ottoman Empire. There are harsher periods and occasional outbreaks of violence, but not generally the same rolling pattern of expulsions seen across medieval Europe.
The modern shift comes later, and in a rather less comfortable way. From the 19th century onwards, European antisemitic ideas start to circulate more widely. Myths that had been distinctly European begin appearing elsewhere, as in the Damascus Affair. By the 20th century, you have nationalism, colonial pressure, and the Israel Palestine conflict all feeding into the mix.
So contemporary antisemitism in parts of the Muslim world is not simply a straight continuation of early Islamic attitudes. It is a blend. Some older assumptions about hierarchy, a significant layer of imported European thinking, and a large dose of modern politics.
Which leaves you with a slightly inconvenient conclusion. There is no single origin story that neatly assigns blame to one civilisation. What you have instead is a recurring pattern. A group that stands out. A surrounding society that expects conformity. A period of tension. And then the familiar slide from different, to difficult, to dangerous.
The explanations change with the times. Religion, race, conspiracy, geopolitics. Each era finds its own way of dressing it up and persuading itself this time it is justified.
And if you listen to how people argue about it now, usually with great confidence and very selective history, you can hear the same pattern playing out. Just with better vocabulary, a few more references, and the same old habit of deciding who to blame before working out why.