R. R. Reno, in his Genesis commentary, calls attention to the “scandal of particularity” that is the call of Abraham. Up to this point, the text deals with a broad look of humanity. The curse of Adam and Eve spreads out to their children, encompassing the entire race.
We have the very first murder, then the founding of the first city by the guilty. The descendants of Cain then forge the first instruments, both musical and of bronze and iron. The narrative sweeps us in the global flood, and after the children of Noah spread out on the new earth, the story of Babel tells us of the origin of all languages and cultures.
These have the feel of legend. The stuff myths are made of. We are reading history from a cosmic perspective.
Since the fall, the story has given us several false starts. How is God ever going to put things to rights? But whatever we think, we expect that whatever God does to begin the reversal of the Fall will retain the same epic, universal feel.
And then suddenly we get to Abraham. One man. One family. And God tells him to just start wandering in the the land of Canaan. Abraham is special only because God says he is, declared with the same voice that brought the cosmos into existence. Why Abraham and no one else? Because God declared it. The theme of election, which is a major theme of the rest of the book, is introduced, and this shift is important.
Because the divine plan, of course, is still universal in scope. Through this one man, the whole world will be saved and the nations blessed. God instills the promise of the future in flesh and blood. And what does this prepare us for?
In other words, Genesis 12:1 is the beginning of the gospel itself, both in form and method.
As will happen time and time again in Scripture, God is the God of the unexpected. He will continuously surprise us.