Brothers and Sisters, The covenant with Abraham is a milestone covenant because the Sovereign Lord reveals what His purposes are regarding the redemption of fallen humans to Himself and that purpose includes Abraham and his family.
First, God reveals Himself for the first time by the name El Shaddai - the Almighty God. He emphatically declares that He alone is all-powerful and he alone can and will perform what he has promised. But He also reveals that His covenant relationship with Abraham requires Abraham to walk before Him and "be perfect." As true believers know, that is an impossible human standard. Yet, within the command is God's promise to accomplish what He requires. Seven promises are graciously pronounced by El Shaddai and taken together, they show that God will not be thwarted in His plan to save a people for Himself. In particular, He promises to make (the now renamed Abram) fruitful and to multiply him. That means that the promises are not merely to Abraham, but explicitly to his posterity as well. And the promises also point us to God's purpose to accomplish through His people what He commanded Adam and Noah to do. Namely, to fill the earth and subdue it and be in close communion and fellowship with our Creator forever. That is, the promise is not substantially about the rocky and dry soil of the Near East. It is about God's eternal purpose to be a God unto His people now and in eternity.
"You shall keep...therefore" appears next in God's revelation to Abraham. Obedience was (and is) the necessary and reasonable consequence of God making a covenant. (Romans 12:1) The word used here is the same that God used in His command to Adam to dress and keep the garden. Adam was to watch over it, guard it, take care of it, pay attention to it. So are Abraham and his children to watch over, guard, take care of and pay attention to the covenant. That is, if we are included in the promises, we are also included in the obligations. This is the only reasonable, rational, logical consequence. And the sign that is established points us to another truth of life in the covenant: the need for casting off the old nature and cleansing. Both are necessary and both are not merely outward forms, but they are spiritual necessities and realities. (Deut. 10:15, 16)
As I mentioned in the opening paragraph, part of God's redemptive revelation is that Abraham's family will be central. Abraham (through Sarah, not Hagar) was to be the father of many nations Abraham pleads for the son that he already has. But that son was conceived and born according to the will of the flesh, not according to the will of God. (John 1:12, 13) El Shaddai decrees that it is through Isaac that the promises will be fulfilled and it is through Isaac that El Shaddai declares that He will be the one who determines who will be the inheritor. This should be a comfort to you because El Shaddai decreed that it is not according to the flesh that we inherit the promises. If you have cast yourself upon the mercies of Christ, then you are a child of Abraham by faith (Gal 3:19, 20), for it is after the manner of our father Abraham that we believe God and it will be counted to us as righteousness. (Gen. 15:6) If you have believed in the Lord, walk now before Him and be perfect according to the imputed righteousness of Christ alone.