In 1994 when this book was first published it was pretty way out to claim that Christianity had influenced what everybody thought of as the older religion. It was obvious that Judaism had influenced Christianity, but the other way round - how could it be? Now it is much more generally accepted that Jews couldn't possibly have lived in Christian lands for over 1500 years without it having some effect.
To give a very simple example, the great Medieval scholar Maimonides wrote, "The custom is for all Jews in Spain and the West, in Babylon and in the Holy Land to spread out mats in the synagogue on the floor to sit on. But in the cities of Edom they don't sit on the floor, they sit on chairs". Now what did he mean by "the cities of Edom"? To the Rabbis it frequently meant Rome, and so here he clearly means Roman Catholic practice, i.e. Christian Europe. And the custom of providing chairs, which Maimonides said spread from Christians to Jews, was soon universally adopted by Jewish communities everywhere.