Every society has that which bonds it: A common ancestry and a system of lineage. Or a common language, or common borders, or governing body. Usually, it is a combination of several factors that mold a mass of people into a single whole. The Jewish people are unique in that they have only a single nucleus—and it is none of the above. All that bonds us is Torah. Nothing else has proven capable of holding us together for more than a generation or two. Nothing else, other than the same...