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 Torah that first forged us as a nation.
From a letter, and on many occasions. See also Likutei Sichot, volume 24, pp.18–19.