Mm, not really sure how to answer. Depends what you'd call Hitler's theories of racial supremecy. I suppose he thought it was scientific.
Good question, actually. I guess for a lot of people, it was religious. Some unpleasant Christians insist the Jews "killed Christ." But the Nazis considered the Jews to be a race, rather than a religion. Thus, they considered you a Jew if you had Jewish parents, even if you were an atheist, a converted Christian, or anything else.
And the Nazis weren't into religion, really. In Nazi Germany, the church was part of Hitler's plan for women (along with cooking and having a lot of Aryan babies), but devotion to the party should outweigh any devotion to the church.
I'd say, principly political. The Jews represented a social class that working class Germans resented. Unemployment was ridiculously high, and people were hungry, and stereotypically, the Jews were the business owners and the bankers that seemed to be getting fat off the decent people.
These businessmen could be conveniently lumped into one ethnic group and used as a scapegoat. I'm sure that religious suspicion attracted many people to the movement, though. It just helped to make the Jews stand out.
Now that I think about it, didn't Hitler consider the Aryans to be God's chosen?
There's an inconclusive answer for you! interesting question. I'm part way through a History degree, but I haven't studied the Nazis since GCSE so forgive the holes in my knowledge.