How did the Germans know who was Jewish?
German officials identified Jews residing in Germany through the normal records created by a modern state. They used census records, tax returns, synagogue membership lists, parish records (for converted Jews), routine but mandatory police registration forms, the questioning of relatives, and from information provided by neighbors and municipal officials.
In territory occupied by Nazi Germany or its Axis partners, Jews were identified largely through Jewish community membership lists, individual identity papers, captured census documents and police records, and local intelligence networks. Germany’s racial laws identified a “Jew” as anyone with three or more Jewish grandparents, regardless of their religious identity or practice. Conversions to Christianity were pronounced illegitimate going back two generations, formalizing and instituting Nazi racial theories.