Lech-Lecha inaugurates the covenant history of Israel: God's threefold promise to Abraham of land, offspring and blessing frames the rest of the Torah. The covenant of circumcision (brit milah) commanded here remains a defining rite of Jewish life, performed on the eighth day. The portion is also foundational for Christianity and Islam, both of which trace their spiritual descent to Abraham's faith and journey.
The rite of circumcision commanded at ninety-nine, performed by Abraham first on himself and then extended to his household including Ishmael (Genesis 17:23-27), predates its restriction in the following generation to Isaac's line, and rabbinic law derives from the eighth-day timing given for later generations (Genesis 17:12) one of the few positive commandments observed even on Shabbat, when most other work is forbidden, reflecting the priority classical halakha assigns to the covenant it marks. The renaming of Abram to Abraham and Sarai to Sarah (Genesis 17:5, 15), each adding or altering a Hebrew letter, is read in rabbinic tradition as more than nominal, a change in essential character accompanying the covenant. Genesis 15:6, 'he believed in the Lord, and He reckoned it to him as righteousness,' became independently significant far beyond Jewish tradition once cited by Paul in the New Testament as a foundational text for Christian theology of faith apart from ritual law, a use classical Jewish commentary reads in its own original context of covenant trust rather than as opposing law and faith.