My understanding regarding why Jews are portrayed in Medieval and Renaissance visual art with horns, has to do with a simple mistranslation of the Hebrew in the Torah. My understanding is that when Moses (our teacher of Blessed Memory) descended from Mount Sinai, it was written in Sh'mot that he was surrounded by rays of light. Apparently the Hebrew word for "rays of light" is written similarly and sounds similar to the Hebrew word for "horns". The word when transliterated is "carnaim", which is apparently in Hebrew a homonym. So while this explains the reason for the original mistranslation, it does not explain why anyone today would hold that Jews have horns. It has been over six centuries since Medieval paintings depicted Jews as having horns, and anybody that is even remotely interested would have learned what the initial mistake was that led to depicting Jews with horns; It would be inexcusable for anyone to make such a mistake today.