What defines a person as a Jew by blood?

A person is defined as a Jew by blood through descent from the promised covenant line of Abraham, Isaac, and Jacob. Specifically, coming out of the 12 tribes makes him a Jew by blood. This lineage, rather than religion alone, establishes Jewish identity by blood.

Les explains that being a Jew involves both religion and blood, but first and foremost a person is a Jew because they come from the promised covenant line of Abraham, Isaac, and Jacob. That lineage makes him a Jew by blood if he has come out of the 12 tribes. This follows the biblical account where God called Abraham out of an idolatrous civilization, as Joshua 24:2 describes Terah, Abraham's father, and Nahor serving other gods. Out of that idolatrous race, God took one man, Abraham, and that began the nation of Israel, commonly called the Jew. So the covenant line through the 12 tribes defines Jewish blood identity.

Is a Jew a Jew by religion or by blood?

A Jew is a Jew both by blood and by religion, but first and foremost by blood. That identity comes from being in the promised covenant line of Abraham, Isaac, and Jacob, out of the twelve tribes of Israel.