Isaac is almost a footnote in the history of Israel. He’s born; he’s almost sacrificed by his father; he marries Rebekah; he buries Abraham; he fathers Esau and Jacob; he almost gives Rebekah to Abimelek; he’s tricked into giving away Esau’s blessing to Jacob; then he reconciles with Jacob and dies, all in the span of a few chapters. Abraham’s story takes up most of Genesis 12-25, from the call of Abraham to his death. Isaac’s story overlaps Abraham’s in chapters 21-25 and by chapter 27, the story of Jacob is starting to overlap that of Isaac. By chapter 28, the focus of Genesis has shifted to Jacob, now on his way to his uncle Laban’s household, where many of his adventures will occur, and we only have a few mentions of Isaac from here on. The only story about Isaac that’s not tied to either Abraham or Jacob is Genesis 26, which is a verbatim retelling of the story of Abraham and Abimelek, from Genesis 20, but unlike his father, Isaac doesn’t get cattle or grazing rights out of the encounter. The incident with Abimelek adds nothing tangible to the story of Isaac, and seems to have been inserted simply to give Isaac an adventure of his own before skipping to the story of his son Jacob.
Even in stories where he’s featured, Isaac takes a secondary role. When YHWH tells Abraham to sacrifice Isaac (something that was never done with Ishmael, the oldest) it’s to test Abraham’s faith. Abraham accepts the request without question, which seems rather odd. Recall that in Genesis 18, when YHWH tells Abraham that Sodom and Gomorrah are to be destroyed, Abraham barters with YHWH to gain concessions for the denizens of those towns, yet here, YHWH is telling him to sacrifice the son he’s waited until his old age to have and Abraham seems totally okay with it. We’re not told how old Isaac is, but at one point, he speaks to Abraham, so he must be at least five or six years old, and maybe as old as eight or nine. Despite this, he doesn’t say anything when it becomes apparent Abraham is about to sacrifice him.
In the ancient world, whenever a town was founded, often the founder would sacrifice his oldest child, usually a son, to become the guardian spirit of the town. There’s a mention of this practice in 1 Kings 16:34, describing the rebuilding of Jericho. In this instance, however, Abraham is not founding a city, as he’s still depicted as a nomadic herder. There are clues to suggest that in the original legend, Abraham sacrificed Isaac, and this fact was altered by the author of Genesis, who was writing after human sacrifice had been abolished. The death of Sarah immediately follows the story of Abraham being tested, and the death of a son she had at an advanced age, combined with all the other factors working against her, might have been sufficiently stressful to hasten her end. If this is so, it placed quite a burden on the author of Genesis to account for an individual who wasn’t originally in the story line.
The solution seems to have been to make Isaac the conduit through which Abraham was connected to Jacob. Either that or there was a tradition among the tribes that Isaac was the father of Jacob but that they were somehow descended from Abraham, so the author of Genesis made the logical leap. In genealogy, it’s often a common error to attribute a child to a nearby family with a similar name. It’s also possible that then, as today, many different people claimed descent from Abraham with no rhyme or reason as to how and Isaac was used by the children of Israel to make their connection. Unlike Abraham, who started out as Abram, and Israel, who started out as Jacob, Isaac is not given a new name, signifying a covenant with YHWH. His “covenant” was through that of his father, and fulfilled by the descendants of his son.
In all probability, these stories are based on authentic legends about Isaac and Jacob, since they don’t show Jacob in the best light, being deceptive in taking his brother’s birthright and later in deceiving his uncle Laban (who pretty much deserved it) into giving away the best of the flock when they divided their assets. Most of the story of Abraham and his descendants through Jacob are part of the “hidden book” discovered by Richard Elliott Friedman in the pages of the Old Testament and chronicled in his work, The Hidden Book in the Bible.