In the 19th and 20th centuries, the question of whether a shared faith or a national identity unifies the Jewish people was the major question of Jewish political thought, with some founders of Reform Judaism representing one extreme and secular Zionists the other. Yet most Jews have insisted that the answer lies somewhere in between. David Goodblatt examines how ancient authors—Jew and Gentile—thought of the Jews, noting that many referred to them by using the Greek term ethnos, which implied a group with both shared ancestry and shared customs:
[T]he connection between [Jews’] behavioral expressions of Jewish identity and their ancestors [found in Greek writings also] may appear in rabbinic tradition. The mid-2nd-century CE rabbi Yosi ben Ḥalafta appears as the author of the statement in the talmudic tractate of Y’vamot that “a convert is like a newborn child.” This could be understood to mean that the convert is “born again,” this time as a member of the Jewish people.
If so, then one branch of rabbinic tradition disagreed. Tractate Bikkurim forbade converts from reciting the prayer formula “God of our fathers.” This indicates that culture is not sufficient for full membership in the Jewish people. A dissenting view cited in the Jerusalem Talmud permitted converts to recite this formula because they could claim the [the biblical patriarch] Abraham as their father or forefather. A probably later source, Midrash Tanḥuma, stated, “Abraham is the father of converts.” While this may mean only that Abraham was the original convert and hence model for all subsequent proselytes, it also could be taken as literal adoption. . . .
The fact that adopting the culture of a group entailed establishing a new kinship relation shows the tenacity of the idea that ancestry and behavior belong together. Further evidence of this concept in ancient thinking about ethnic identity is how Greeks commonly described the culture of an ethnos as “ancestral” as in “the ancestral customs” or “the ancestral laws.” . . .
In sum, the connection of behavioral expressions of Jewish identity to the ancestors reminds the reader that ancestry and culture normally go together. Further evidence for the ongoing significance of ancestry in Jewish identity appears in Tractate Sanhedrin, [where] Rabbi Abba ben Zavda (late-3rd early-4th century) comments, “Even though he has sinned, he is [still] an Israelite.” . . . Still more evidence on the ongoing role of ancestry comes from the phenomenon of the “Godfearers”—Gentiles who adopted Jewish practices but refrained from becoming full-fledged Jews. What they lacked was adoption by or absorption into the Jewish people.
Read more on Ancient Jew Review: http://www.ancientjewreview.com/articles/2018/10/24/ancient-jewish-identity