What was Israel's wrong assumption in Romans 9 about being God's covenant people?
Israel wrongly assumed that being physical descendants of Abraham and children of the covenant, prophets, and promises automatically guaranteed God's promises, so they had nothing to worry about. But Paul shows not all Abraham's seed are children of promise; Ishmael and Esau were excluded, and entering God's promises still required faith.
Les explains that Israel became puffed up and arrogant, reasoning that because they were the children of the covenant, the prophets, and the promises, they had nothing to worry about. He says Paul blows them out of the saddle by insisting they cannot enter God's promises merely through right genealogy; they must enter by faith. Les also points back to Romans 9:7: not all who are Abraham's seed are children. Ishmael's and Esau's offspring were physically Abraham's, but not according to the promises, because the seed was called in Isaac, referring to Christ.