Because anti-Semites are always pretty open about why the dislike/hate/want to see dead Jewish people. and because they give many answers as to why, people tend to think anti-Semitism is a complex issue.
If you travel in the far east you won’t find anti-Semitism in Hinduism, Buddhism, Daoism, Jainism. Sikhism the Bahai or any of those religions but you will always find it in Christianity and Islam. These last two religions teach that the Jewish people murdered god on earth (Christians) and that idolaters and Jews are the eternal enemy (Islam). The inherent anti-Jewish writings and teaching in these two religions is the foundation of all anti-Semitism in the west and near-east.
When people start off being prejudiced towards another people those people cannot do anything without tapping into that prejudice. We have long known that hatred is taught and there is no clearer example of that fact than in how Islam and Christianity has educated its people about Jewish people and how their religion supplants Judaism.
Secular society dropped all of that but for their treatment of, and thoughts about, Jewish people.