Ethnic Jews are still descendants of Judah, the fourth son of Jacob (Israel), no matter what religion they may profess. AFAICS, Adolf targeted Jews by both ethnicity and religion.
In the Nazi ideology as expressed in the Nuremberg laws, being Jewish - and therefore whether you were targeted for death - was determined by one's genealogical tree. If you had at least one eighth Jewish ancestry, you were Jewish as far as they were concerned, regardless of your religious or cultural or ethnic identification.