[Alouette]

"Purity of blood" (hist.): quality of being descended exclusively from Old Christians, without having any ancestor (who was) Jewish

But if somebody traced his ancestry all the way back to the original apostles, wouldn't that make him the descendant of Jews?

[Verginius Rufus]

I don't know if anyone ever claimed descent from the apostles...St. Peter was married but we never hear mention of any children he might have had.

Probably a great many Spanish Christians had some Jewish ancestors at some point in their family tree, but I guess if it was a remote ancestor no one knew about it didn't count.