Toland said this was specifically the rule so that Jesus and Hitler both qualified as Aryan.
Hitler worried that he might be a quarter Jewish by blood, so this law cleared him.
And since Jesus's father was God (not Jewish) and Jesus was a Christian, therefore Jesus was not Jewish. (All of which is silly, but the Nazis created this rule in an attempt to convince Christian Germans that Nazis were not anti-Jesus.)
Well, that's what I remember Toland writing.
Toland was wrong. I got my information from a research book titled “Hitler’s Jewish Soldiers: The Untold Story of Nazi Racial Laws and Men of Jewish Descent in the German Military” by Bryan Mark Rigg. According to the legal test conducted for the Nuremberg Laws, if didn’t matter if you’d converted to Catholicism, if you even had one grandmother that was Jewish, you were considered a Mischling, a mixed blood, and a Jew. And there were different classes of Mischling, but just changing your religion didn’t make you Aryan.