The Crusades gave birth to the papal law of limpieza de sangre—cleanliness of blood—for which the Inquisition was established by the Church to investigate and determine. Before this time the concept of biological race based on "blood" is not known to have existed as law or taboo in Christian Europe or anywhere else in the world. As scapegoating and suspicion of Conversos (Jews who had converted to Christianity) and Moriscos (Muslims who had converted to Christianity) intensified over several centuries in Christian-controlled Spain, the doctrine of limpieza de sangre was popularized. It had the effect of granting psychological and increasingly legal privileges to "Old Christians", both rich and poor, thus obscuring the class differences between the landed aristocracy and land-poor peasants and shepherds. Whatever their economic station, the "Old Christian" Spanish were enabled to identify with the nobility. [...]

This cross-class mind-set can be found as well in the stance of descendants of the old settlers of British colonization in North America. This then is the first instance of class leveling based on imagined racial sameness—the origin of white supremacy, the essential ideology of colonial projects in America and Africa.

— An Indigenous Peoples' History of the United States (ReVisioning American History) by Roxanne Dunbar-Ortiz, Roxanne Dunbar Ortiz (Page 37)