He also writes this, in the chapter, "Cashing in on Heaven:"
"Ideas can affect history when used by people to define reality and inspire social action. Ideas have an important feedback upon the very conditions that give birth to them. They acquire material force when they trigger sentiments and galvanize minds. But adherents of 'philosophical materialism' would argue that ideas do not arise of their own accord as disembodied self-generating entities divorced from human transmission and artifice.
Most religious believers reject this view. The hold that there do exist certain purely ideational forces, such as spirituality the will of God, and the devil's machinations, and these act -- independently of human imagination -- directly from the ethereal realm onto the material world.'
In his chapter on anti-Semitism, her writes: "Anti-Semitism was used repeatedly by ruling circles to distract the populace from their real grievances about land, taxes, and tithes Better the people should storm the synagogue than wreak their fury upon the manor, the castle, the monastery, or the cathedral wherein resided their real exploiters, their fellow Christians. The officially proscribed Jew served as a convenient scapegoats, blamed for plagues, pestilence, poverty, famines and other supposed manifestations of divine displeasure.
Throughout Christendom, Jews were saddled with an array of legal and social disablities that eventually stigmatized them in the eyes of Christians: special taxes, forced ghettoization, confiscation of property, and the burning of synagogues. Jews were banned from public office and most professions. They were forbidden to own farmlands or engage in export and social contact between Christians and Jews. They were occasions when Jewish children were forcibly removed from their families and handed over to Christian households or monasteries for conversion."
pp 57-58
Basically, then, Parenti isn't peddling idealism. Nor is he denying that ideas obtain a material influence on the world. The stigmatization of the Jews results from a material set of circumstances which "saddled them" with "legal and social disabilities." Those circumstances are explained simplistically, to me, but the naive or mechanical marxism at work in Parenti's text traces these events as ideas that were propagated by the ruling classes in order to obtain, maintain,and galvanize their rule, and keep people from figuring out who their real enemy was.