The German Nazi ideology was expressed through the Nuremberg Laws of 1935 which interpreted the inclusion into the "German people" community through racial lenses aimed at protecting the purity of the German blood, (sic). Jews could not become part of the German people and were stripped from proper German citizenship and subjected to various legal limitations.

Their identification as Jews was not determined by their faith but by their ancestry and could not be changed. Special case was the people of mixed origin, the Mischlinge, which dependent on the degree of Jewish anchestry could or could not be considered German. If one was to challenge his category, he would need to prove, through genealogy research, of his non-Jewish origins.

So, it was impossible for Jews or even Christians of Jewish ancestry to become Aryan Christian's unless forging state documents.

source: the actual Laws of Nuremberg but also Saul Friedlander's work displays the length to which Nazi's went to create a legal framework to "legally" remove the protections citizenship offered to Jews which proved a key point to later transforming them from non-citizens, to non-humans.