But for all of this sibling-type rivalry, Christian theologians used biblical Judaism, rather than nascent rabbinic Judaism, as their point of reference. It was from the Hebrew Bible that they consistently drew proof texts to prove the validity of Christian teachings. In this sense Judaism played more the role of a mother than a sister. Indeed, a major aspect of ChristianJewish relations in the twelfth and thirteenth centuries was the complicated response of Christians when they realised just how much post-biblical Jewish material had been produced and how important it was to contemporary Jewish life and thought. For centuries Christian theologians had been so intent on proving that Christianity had superseded Judaism that they had assumed Judaism had ceased to develop after 70 ce. They were strengthened in this view by the Augustinian concept of Jewish witness. And this takes us to the leading theologian to provide Christianity with definitive concepts concerning the relationship between Christianity and Judaism.