The classic (theological) argument is that the Jews were responsible for the execution of Jesus Christ at the hands of Pontius Pilate through their passivity. However, I'd downplay the importance of theology here, as medieval antisemitism (yes, an anachronistic term, but useful nonetheless) tended towards being very popular (as in, of the people) in nature, which precludes the possibility of it being truly motivated by theological arguments since the average peasant would not have had such a sophisticated grasp of dogma. That's not to say that there weren't examples of antisemitism on the part of the nobility or the clergy, but on the whole, those acts of anti-jewry which occurred at the top tended to be more for economic reasons (where the crowns of England, France et al. confiscated the assets of jewish moneylenders to finance their wars, or the Church confiscated jewish land to increase their estates) than a truly religious fervour. It was only after Martin Luther's Von die Juden und ihren Lugen (On the jews and their lies, where he calls the synagogue a defiled whore and for jews to be burnt, amongst others) that a cultural and religious antisemitism comes into play: Heinrich Himmler actually called Luther "The Father of modern Antisemitism", interestingly enough. Medieval antisemitism tended to take on the same sort of superstitious and ignorant colour that we associate with their treatment of witches: accusations of eating christian babies, being associated with the devil, poisoning wells to cause the black death, and so on. The key here is the fact that they were Christian only in name: though they were certainly by affiliation Christian, their belief systems combined what little they could take out of the clergy's obfuscations with the oral traditions of earlier pagan beliefs and a lot of evolving folklore, the latter of which giving us the dwarfish caricature of gold-loving, narrow minded, petty evil beings we see in Nazi propaganda, Wagner's Der Ring des Nibelungen and even in Tolkien's Lord of the Rings. So in short, their justification for anti-jewish sentiments were borne out of prejudice, a fair bit of jealousy (usurers made a substantial amount of money after all) and their lack of grasp of Christianity itself.