What exactly did Nietzsche mean by "Judean slave morality"? I've seen antisemitism explained as from a number of Jewish beliefs/practices. Generally the understandings of the bit of Judaism being discussed is mangled, and it's often used as an explanation of how antisemitism isn't really the fault of the bigots. It's a victim-blaming canard more than a serious bit of history.

How true is it that conflict with religion was a factor in antisemitism, both during and before the Nazi reign?

It isn't, for the most part. You were subject to Germany's antisemitic laws if your parents were Jewish, even if you were a baptized Christian. That said, if you had mixed ancestry Jewish religion might make you classified as a Jew, and certainly negative attitudes towards Jews drew off the history of religious bigotry against Jews in Germany.