The NUREMBURG RACE LAWS-
The Nuremberg Laws, as they became known, did not define a "Jew" as someone with particular religious beliefs.
Instead, anyone who had three or four Jewish grandparents was defined as a Jew, regardless of whether that individual identified himself or herself as a Jew or belonged to the Jewish religious community.
How the Nazi's defined "Jewish Ancestry"....
http://www.historyplace.com/worldwar2/timeline/nurem-laws.htm
Many Germans who had not practiced Judaism for years found themselves caught in the grip of Nazi terror.
Even people with Jewish grandparents who had converted to Christianity were defined as Jews.