Genesis 17:7 (ESV) … “And I will establish my covenant between me and you and your offspring after you throughout their generations for an everlasting covenant, to be God to you and to your offspring after you.”
The Lord now adds to the great promise of kingship what are seen in the OT as even more valuable blessings. The covenant is not to be just between God and Abraham but between God and Abraham’s descendants after him.
Hitherto he has been promised descendants (13:16) and the land has been promised to them (13:15), but this is the first occasion that the covenant is extended to include Abraham’s seed. This makes the covenant with Abraham like the Noahic covenant, as do the phrases “I shall confirm my covenant … for an eternal covenant” (9:9, 11, 16).
Though the land had been promised in perpetuity (13:15), this is the first time that the Abrahamic covenant has been described as “eternal” or that the covenant formula היה לך לאלהים “to be your God” has appeared.
This latter phrase, used twice here and not again till 28:21, expresses the heart of the covenant, that God has chosen Abraham and his descendants, so that they are in a unique relationship: he is their God, and they are his people (cf. Exod 4:16; 6:7; Lev 11:45; 26:12, 45).
What we see in this OT story is a shadow of what Christ promises the NT believers. We receive the promise of the eternal from Christ as well as knowing that he is our Lord and we are his people. God the Father, The Son and The Holy Spirit is the one true God of promise and even more exciting, the keeper of those promises!