Boyarin again ( Dying for God: Martyrdom and the Making of Christianity and Judaism (Figurae: Reading Medieval Culture) , 9), suggesting a linguistic paradigm for understanding the divergences and interactions between Christianity and Judaism:

“Separate languages . . . are merely artifacts of the official canonization of a particular dialect as the official language of a given group . . . . If one were to travel from Paris to Florence speaking only the local dialects in each town or village, one would not know when one had passed from France to Italy. There is no linguistic border ‘on the ground.’ The reason we speak of French and Italian as separate languages is precisely because the dialect of Paris and the dialect of Florence have been canonized as the national languages. Similarly, I would suggest, social contact and the gradations of religious life were such that, barring the official pronouncements of leaders of what were to become the ‘orthodox’ versions of both religions, one could travel, metaphorically, from rabbinic Jew to Christian along a continuum where one hardly would know where one stopped and the other began.”

One should add, of course, that, hard as the boundary may be to identify, one would eventually be conscious of having crossed it. Still, Boyarin is right to emphasize that cultures and languages are products of acts of authority.