
I think a lot of the history of antisemitism at least in Europe goes back to the status of Jewish people as a “middleman minority”. It’s very common that when an ethnic group becomes associated with trade, they are easy scapegoats. The run of the mill people will perceive them as greedy or untrustworthy, and those in political power can use them as an easy scapegoat.
In Europe this position was taken up by Jews, as due to Christian prohibitions on charging interest they were a convenient tool for the broader society, but were simultaneously easy to segregate and limit to keep them from branching out of this role too much and were easy to expel or persecute whenever convenient
In other societies other minority groups take up this position and are frequently subject to persecution during periods of unrest. In Southeast Asia its Chinese people, in east Africa it was historically Indians, in west africa its Lebanese. These are groups which have historically and in modern times been subject to distrust and periods of extreme violence
That’s part of the theological justification for it. A lot of it I think goes back to Rome and Rome’s attitude towards itself and Jews. Many Jewish people were scattered throughout the Roman Empire. As Christianity spread among Romans, it would not be appropriate to blame Rome for Jesus’s death. We see this in the sanitizing of Pontius Pilate. So who to blame instead? Well there’s a convenient already distrusted minority group spread around Europe.
And sometimes you get theological justifications just shoved in way after persecution started. In southern France, for example, there was a persecuted minority called the Cagots. Local legend claimed they were descended from the carpenters who built the cross Jesus was crucified on, providing religious backing for their persecution. That’s obviously ridiculous, but people believed it.