Q. Why do medieval texts think the Jew is a devil?
He makes it clear that whoever did not believe in Jesus was considered to belong to the devil. Though the early Christians were Jewish, there developed a view, e.g. in the Gospel of John, that the devil was the father of the Jews; the book of Revelation called a Jewish place of worship a synagogue of Satan.
Neither Satan nor the devil was mere metaphors; the medieval church took them literally and personified the Jews as diabolical or satanic. The belief was that the devil had a stench, therefore the Jew smelt. Satan had horns and an ugly beard: so did the Jew.
Some literalists of our own day are still steeped in these medieval superstitions and no amount of rational explanation or persuasion helps and even Muslim propaganda utilises this primitive thinking. Fortunately there are many moderates within both groups who are adamant that primitive prejudice must go.