7 And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee.
9 And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations.
10 This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.
14 And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
The promises that God had for Abraham were not for Abraham only. In order for the promises to come true, they would need to be reiterated to all of Abraham’s willing descendants. Abraham wasn’t going to have a nation of children immediately in the next generation, so his children, grandchildren, great-grandchildren, and beyond would all have to be included in the promise of raising a great nation and possessing the land of Canaan.
This is a very special sort of perpetual blessing. Usually when I think of a promise from God what comes to mind is a personal thing. I receive my promises from God and you receive yours, and just because God gives one person one thing does not mean He is likely to give the same thing to someone else. But, evidently, sometimes a promise from God is made to a group, or even to an entire line of posterity.
And this is a foreshadowing of the New Testament covenants that Jesus would later introduce. Covenants that would be offered even more liberally, to the entire human race instead of just one family line!
Though not necessarily all of Abraham’s descendants were going to be part of his covenant, and not necessarily all of humanity will be part of the New Testament covenants either. All of Abraham’s descendants could be, but it would depend upon them showing God a specific sign, which was being circumcised, and following a specific law. And the New Testament covenants are also fully entered into by showing God certain signs, such as baptism, and by following Jesus’s higher law.