Anyway, I would say the Nazis were persecuting the Jews more on the ethnic component than the religious one.
I highly recommend you read
Constantine's Sword, which is a very detailed and engaging account of European antisemitism from the Roman conquest of Judea through the Holocaust.
You really cannot separate the ethnic and religious components of antisemitism. The Nazis played on the religious basis for antisemitism for their political gains. And for much of history the ethnicity and religion of the Jews was interchangeable.
In fact, the European concept of Jews as an ethnicity and not a religion really began as an offshoot of the Catholic Inquisition, where even after Jews were forcibly converted, the idea sprang up that the conversos were still secret or Cryptojews worthy of persecution and that their sin of killing Christ almost became a second original sin that applied only to them.
So in the 1500's, this sort of idea developed that Jews were always different, even if they tried to assimilate. At the same time as this idea was developing, Jews were fleeing Spain into Germany and Eastern Europe.
And this was exactly the time that Luther was writing. Luther at first reached out to the Jews, thinking that the only reason Jews hadn't converted was because of their treatment by the Church. So he felt tolerance would lead to conversion. But, when the result was merely that Jews flocked to Germany because it was tolerant, but did not convert, Luther decided to jump back on the antisemitism bandwagon and in 1543 wrote "On the Jews and Their Lies" which, while certainly more openly venomous in its antisemitism, represented more or a return to traditional Catholic antisemitism.
His attitudes -- fueled by religion -- seeped into the German consciousness and helped create Nazi antisemitism, and then got wrapped up in Nazi racial theories.
In short, you can't separate the ethnic and religious components of Nazi antisemitism. They proceeded in a straight evolutionary line.
If every Jew in Europe had converted to Christianity would the Nazis have spared them? The Nazis were obsessed with blood, not belief.
Except that's no different than the conduct of the Church, who continued to persecute conversos even after they converted. By the 1930's religious antiemitism already had a racial component ingrained within it for more than four centuries.
None of which means the Catholic Church wasn't anti-semitic, of course. But I don't think they invented it
They didn't. They inherited it from pagan Rome. And in that sense, one could say that Catholic antisemitism was at its heart ethnic in basis -- in that the Romans hated Jews because of their obstinant refusal to accept Roman rule over Judea, rather than any religious disagreement (although there were those too). The Church simply took pre-existing Roman hatred of Jews and gave it a religious justification. And then the Nazis took Christian antisemitism and gave it a racialist justification.
The Nazis didn't create antisemitism either. But they inherited it from the Lutherans, who got it from the Catholics, who got it from the pre-Christian Romans. Each step in the chain added to the antisemitism, making it uniquely theirs, by building on the foundation set by the antisemites who preceded them.
I think you're seeking too much religious reasons behind this. The Nazis sought cool symbols. The eagle was not only a symbol of the (pagan) Roman Empire, the military superpower of its time, but also of Germany, both the Holy Roman Empire (aka the First Reich) and the Wilhelminian Empire (aka the Second Reich) and still of the FRG.