I learned a while ago that antisemitism existed even before it was used as a scapegoat tactics by the Nazis during the Holocaust and that the antisemitic philosophies where not added by Nietzsche but were added by his sister and her husband who were both antisemitic.
However, I am unable to find out exactly why anti-semetism appeared and became a prevelant philosophy for so long.
There are some conspiracy theories out there that claim that the Freemasons did this (which is so secretive that I have no clue what the Freemasons want or why) or the Illuminati and so on. But of course, these are conspiracy theories so I take them with a pinch of salt
This is a huge topic that takes a lot of unpacking, but the short answer is that anti-semitism has been around for pretty much as long as there have been Jewish people. This has taken on a variety of forms throughout the years, but I'll give an overview of some of the main anti-semitic issues in Christian Europe.
"The Jews control the money" is a long-standing slur that has some amount of truth to it. The bible has a proscription against "usury," which was interpreted by many Christians to mean charging interest of any kind on a loan or similar financial transaction. Since there was still a market for loans, Jewish businesspeople, who had no religious objections to charging interest, became well-known as bankers. Not only would they have held the debts of many, many people, they would have been doing something that most saw as immoral and sinful, which stoked anti-semitism.
In the bible, Jesus is brought before the people of Judea by Pontius Pilate, and they are given a choice as to whether to set him or a dangerous criminal free. They choose the other guy. As Pilate washes his hands, according to the Gospel of Matthew (Chapter 27, Verses 24 & 25):
When Pilate saw that he was getting nowhere, but that instead an uproar was starting, he took water and washed his hands in front of the crowd. 'I am innocent of this man’s blood,' he said. 'It is your responsibility!' All the people answered, 'His blood is on us and on our children!'
This has been the basis of the charge that "the Jews killed Jesus," conveniently shifting the blame away from the Romans.
There have been all kinds of wild tales told about Jewish people, from poisoning of the wells during the plague to kidnapping and torturing (and possibly eating) good Christian children for their rituals. I'm not sure you'll find a "start" of anti-semitism, but those are at least a few factors that have led to their ostracization.