Not really a historian but I enjoy theology specifically Christian theology and the main arguement used to reconcile these two was 2 fold one your standard “the Jews murdered Christ” line of thinking which in the eyes of many illiterate peasants, who probably didn’t know much about the Old Testament and only the basics of the new, considered reason enough for antisemitism. The secondary reason theologically is a bit more complex, the idea was that in the Old Testament the Jews were Gods chosen people from whom the messiah, Jesus Christ, would emerge however many medieval Christians believed that in the New Testament and hence Christians were God’s new chosen people and thus Jews were actively rebelling against Gods will by not converting to the new true religion of Christianity and continuing to remain in the old ways. Historical development that occurred soon after Christ such as the Roman sack and destruction of the Jewish temple seemed to many early Christians and later medieval ones to prove that God was angry at the Jews for not following Christ and thus if God was angry it was only proper for Christians, God’s new chosen people to be angry aswell. Hope this provides some insight