After identifying the social and political origins of Basque nationalism in his books El milenarismo vasco (1981) and El escudo de Arqui loco (2001), the author argues here that the ultimate historical and ideological source for the pure-blooded, "socio-religious" understanding of Spanish identity (which reached its most extreme and finished expression under Franco's dictatorship) is the same as that of Biscayan ethnicity from the 16(th) century onwards. This source must be found in the rabbinical thought about Jewish identity that emerged in the Babylonian exile after the destruction of the kingdom of Judea in the 6(th) century B. C. and is revealed by a significant part of the Old Testament.