What does all of the Old Testament rest on from Genesis 12 onward?
From Genesis 12 onward, all the rest of the Old Testament rests on the Abrahamic covenant. That covenant laid out the prospect of a royal family, a coming benevolent King, and a righteous, fair, and glorious kingdom, with later covenants, including the Law and the promise to David, standing alongside it.
Les explains that David becomes the beginning of the royal family that will lead not only through generations of this existence but all the way into eternity. Out of the line of Abraham and David would come this government, a monarchy rather than a democracy, ruled by the King of Kings and Lord of Lords. This King will not be a despot or totalitarianist but benevolent; His kingdom will be righteous, fair, and glorious. This prospect was laid out in the Abrahamic covenant. Therefore all the rest of the Old Testament, from Genesis 12 right on into the New Testament, rests on that covenant. There are other covenants as well: God makes a covenant with Moses called the Law, and a covenant with Moses and Israel that though uprooted they will be brought back to their homeland; along with that, He promises David a royal family leading up to the King and the government. Les then directs attention to Isaiah chapter 9, verse 6.