The Jews are a minority, and as is the human way, we blame things we don't understand. Plague, crop failure, unexplained deaths; all could be blamed on "jewish magic" or "jewish treachery". In medieval and renaissance Europe, the Jews often found themselves in a position as a powerful but distrusted group. The various Muslim rulers of Spain often put Jews in a position of power, as they would have to rely on the local authority to keep control of the populace. When the Muslim rulers fell, the Christian conquerors saw the Jews as collaborators. Lending to a fellow Christian with interest was a practice banned by the Church for centuries, but it did not apply to Jews; many Jews found themselves as bankers, not exactly a likable group. Things got especially worse for them during and after the Crusades. Urban II would use terms like "enemies of Christ" to describe the Muslims in Palestine, but the everyday peasant has never met a Muslim. He does know that "Jews" killed Jesus, though, so clearly they are enemies of Christ as well. Anti-Semitic mobs rose in popularity in the 11th century.