AskHistorians

Antisemitism is an ancient prejudice. Reuther’s Faith and Fratricide is a good work that traces the conflict between Judaism and Christianity all the way back to the period where Christians were still Jews.

A big justifier for medieval antisemitism was the idea that the Jews had killed Jesus. See Matthew 27:22-25

Pilate saith unto them, What shall I do then with Jesus which is called Christ? They all say unto him, Let him be crucified.

And the governor said, Why, what evil hath he done? But they cried out the more, saying, Let him be crucified.

When Pilate saw that he could prevail nothing, but that rather a tumult was made, he took water, and washed his hands before the multitude, saying, I am innocent of the blood of this just person: see ye to it.

Then answered all the people, and said, His blood be on us, and on our children.

(Keep in mind that this story makes absolutely no sense, and was probably a later addition as Christianity became more “Romanised” and wanted to shed its more revolutionary, anti-Empire roots.)

The Catholic Church only stopped blaming Jews for killing Jesus with the Second Vatican council in the 1960’s

Pogroms throughout the Middle Ages and afterwards were fairly common. Medieval Christians believed that Jews poisoned wells to spread the Black Death, that Christian infants were kidnapped for their blood to be used in matzo balls, that Jews secretly had horns… Jews were often forced to convert, see during the Reconquista where Isabella and Ferdinand ordered an expulsion from Spain of all Jews who refused.

Hitler wasn’t just randomly picking Jews. Hating Jews was an entirely mainstream thing for most of the past two millennium.