"WHAT'S THE CAUSE OF ANTISEMITISM?"
Jews maintain their distinct religious, ethnic, and cultural identity while living among larger, more powerful groups.
That's ultimately it.
The Hellenist and Roman empires all tried to force Jews to abandon their traditions and their belief in the one God and embrace polytheism - and were real mad when the Jews refused and fought back. After Rome destroyed Judea in 70 CE, the Jews were then dispersed into other territories and nations - and those in turn grew suspicious and hateful towards a population of immigrants who lived among them but would not be exactly like them, who would instead live in ways that suggested that the preferred beliefs of those other nations might actually be wrong. This hatred was there before the development of today's countries and political ideologies, and so it was absorbed into them and shaped them as they developed - hence the modern stripes of antisemitism we see in Europe, in the Middle East, among the far right and the far left.
As outsiders-among-us, already associated with what-we-are-not, the Jews became targets for ALL subsequent hatreds. In about ten seconds, you can find examples of people blaming Jews for inventing or controlling all of the following:
* Communism AND Capitalism
* Socialism AND Fascism
* Leftist Globalism AND Right-wing Nationalism
* Immigration AND Immigration bans
* Miscegenation AND Racism
* Sexual repression AND Pornography
* Criminals AND Police
"SHOULDN'T CHRISTIANS HAVE LIKED JEWS BECAUSE OF BIBLE / JESUS?"
Christianity, and to a lesser extent Islam, fully embraced for centuries the concept of supersessionism, also known as replacement theology. The idea was that Judaism was old, incomplete, and inferior, something to be corrected and replaced and at all times utterly dominated by the newer culture. Mainstream Christian teachings utterly loathed Jews for centuries and were largely oriented around inventing new crimes to accuse them of (ritual child sacrifice, poisoning wells, causing plagues, etc). The notion that "the Jews killed Christ and should be condemned for it" was 100% standard mainstream teaching for nearly 1,600 years. It was only officially reversed by the Second Vatican Council in 1967 - and that was massively controversial at the time, and is not universally accepted today.
"ARE JEWS WHITE / CAUCASIAN?"
Jewish people are an ethnic group, that mostly practices the religion of Judaism. "Aren't Jews just white people?" is a question that could only be asked in a major American city after about 1970. For the rest of the world and the rest of history it would be nonsensical. The American obsession with a color binary is not normative, not a default that the rest of the world has to adhere to; walk around in Europe and you will quickly be told that Germans and Finns are two entirely different ethnic group, as Russians are from Georgians, as Czechs are from Slovaks, as Magyars are from Serbs, etc. Well into the 20th century it was commonplace for Jews to be depicted as a dark, hairy, Asiatic race. Pale-skinned American Jews in 2023 have benefited from white privilege for about two hours; it is conditional, was by no means predictable they would ever have gotten it, and could just as easily be taken away. There's also the question of how "white" a visibly traditional and less assimilated Jewish person is, with the hairstyle and clothing that makes them an obvious and constant target for hate crimes.
And that's just for pale-skinned Jews. Because of how immigration worked in America (WHOLE DIFFERENT AND ALSO BAD CONVERSATION), most American Jews wound up Ashkenazi, from eastern Europe. But there are plenty of Sephardim from southern Europe, and Mizrahim from the Middle East, and Bnei Menashe from India, and Beta Israel from Ethiopia, and many many others.
Try to get someone to address whether they see Arabs as a real group and Jews, even Ashkenazi Jews, as just white people. It is a conversation worth interrogating.
"WHY WEREN'T ATHEISTS / AGNOSTICS PERSECUTED?"
They were, if they developed - but there was no such thing as an established atheist / agnostic culture in which children would be raised. All society was oriented around either Christendom or the Islamic ummah. Individual people could fall away from belief, and maybe recruit another here or there, but it was hard to organize and easy to snuff out, literally one at a time. And how can you REALLY tell if someone is an atheist? You can't read their mind while they pretend to pray. It is much easier to seek out, and find, Jewish prayer books or ritual objects, or a circumcision, and find the guilty that way. Jewish identity and traditions are passed on from parent to child - there was always a visible society existing as an alternative within the larger dominant one.