The doctrine of Israel's chosenness rests on a small number of biblical texts that later tradition built into a full theology. Exodus has God tell Moses at Sinai that Israel, if it keeps the covenant, will be a treasured possession among all peoples, am segulah, and a kingdom of priests and a holy nation. Deuteronomy repeats the language and ties it explicitly to smallness rather than merit: God did not choose Israel because it was the most numerous of the nations, for it was the fewest of all peoples, but because of love and a promise kept to the patriarchs.
Rabbinic and liturgical tradition built steadily on this base. The blessing recited before reading the Torah thanks God for having chosen Israel from among the peoples by giving the Torah, tying chosenness directly to the gift and burden of the law rather than to any inherent superiority. The Aleinu prayer, closing every daily service, thanks God who has not made Israel like the nations of the lands and has not made its portion like theirs, language that has drawn both traditional affirmation and, in some liberal prayer books, revision.
Classical sources consistently pair chosenness with responsibility and warning rather than privilege. The prophet Amos has God address Israel in the sharpest terms available in the canon: only Israel has God known of all the families of the earth, therefore God will punish Israel for all its iniquities, making election the ground of a stricter accountability, not an exemption from it. This reading, chosenness as a heavier yoke rather than a lighter one, runs through the tradition's classical self-understanding even as later movements would argue over whether the doctrine could survive into a pluralist age at all.