John Sailhamer:
The theme of Abraham and his descendants marking a new beginning in God’s plan of blessing is developed in a number of other ways as well in Genesis. Most notable is the frequent reiteration of God’s “blessing” in 1:28 (and 9:1) throughout the narratives of Abraham and his descendants (e.g., 12:1-3; 13:15-16; 15:5, 18; 17:6-8; 22:17-18; 25:11; 26:2-4; 27:27-29; 49:28). The “promise to the fathers” is none other than reiteration of God’s original blessing of humankind (1:28).
To make this clear the author has given a representative list of “all humankind” in chapter 10 according to their “families” (10:32) and has shown how their dispersion was the result of Babylon’s rebellion (11:1-9). These same “families of the earth” are to be blessed in Abraham and his seed (12:3). Abraham is represented here as a new Adam and the “seed of Abraham” as second Adam, a new humanity. Those that “bless” him, God will bless; those that “curse” him, God will curse.
The way of life and blessing, which was once marked by the “tree of the knowledge of good and evil” (2:17), and then by the ark (7:23), is now marked by identification with Abraham and his seed. The identity of the “seed” of Abraham will be one of the chief themes of the following narratives.
The Pentateuch as Narrative, p. 139-40, my emphasis.
The Apostle Paul:
16Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings [seeds],” referring to many, but referring to one, “And to your offspring [seed],” who is Christ.
…
23Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. 24So then, the law was our guardian until Christ came, in order that we might be justified by faith. 25But now that faith has come, we are no longer under a guardian, 26for in Christ Jesus you are all sons of God, through faith. 27For as many of you as were baptized into Christ have put on Christ. 28There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. 29And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
