In order to understand this essence of Jewishness, let us return to the more basic definition in the Torah itself.
The Bible calls the Jews ―the House of Israel‖ or ―the House of Jacob, meaning that the Jews are essentially and principally a family.
This family began as the small group of the offspring of Abraham.
With natural increase, they became a clan and then a tribe and ultimately a nation.
But in spite of its extraordinary proliferation, this tribe, and eventually the nation, remained what they had always been—a single family.
They are not really a religion or a nation; they are a family.
The Jews are thus the people who belong to this family of old, the House of Israel.
And almost all that they do, in religious matters or in national affairs, or in any other realm of action, is derived from their being members of the one family.
The relation between them and others, and even more so, the relation among themselves, the way they see themselves, is basically that of a family.
It is from this sense of family that other aspects of their existence function.
The connection of a Jew to his people is not dependent on his being in any particular place or on some religious or cultural tie.
It is rather a matter of belonging to a family.
This kind of belonging to a family is different from other forms of belonging to a social group.
It is not voluntary, being a fact beyond choice that does not necessarily involve anything more than the practical and emotional relations of kinship.
A person need not love or respect the other members of his family.
He may distance himself as much as he pleases, and even act against them, but he can never cut the intrinsic bond to them.
A Jew does not become a Jew, he is born one; his belonging to the House of Israel makes him a Jew.
The acceptance of the Jewish faith, the keeping of the religious mitzvot, or any other way of being involved with Jews or Judaism is a matter of choice, and like every such matter of free choice it depends on the personality and the will of the individual, but the belonging itself is not dependent on the personal will.
–Rabbi Adin Steinsaltz
From We Jews by Rabbi Adin Steinsaltz