After all, KK's world has both Islam and Christianity, and you can't have either without Judiasm.
As KK herself pointed out in Deryni Magic
. She said something along the lines of "Many readers have queried the apparent absence of Jews in Gwynedd. One should never assume that absence of proof is proof of absence...when it comes time to tell a story in which being Jewish is important, as opposed to merely human or merely Deryni, we will see Jews, never fear."
I'm probably misquoting a bit as I'm doing it from memory (too lazy to go grab the book and find the citation).