God always keep His promises. And when we read that God did to Sarah as he had promised, this is talking about more than what we read in Genesis 18:9-15. It is also talking about Genesis 17, and Genesis 15, and Genesis 12, and Genesis 3:15. God is moving the story of redemption forward through the child of promise, Isaac.
And we see God’s grace at work beyond just the birth of Isaac. Though run out of Abraham’s house, Hagar and Ishmael are protected by God. Abraham continues to prosper, and kings seek to make peace with him. Indeed, God has blessed his chosen servant above what he could ever ask or think.
Then we get to chapter 22. We don’t know how much later this is, but Isaac is old enough to reason with his father about the sacrificial lamb (v. 7). By the next chapter, Isaac is 36 years old. Point is, Isaac is well aware of what is going on when Abraham is set to sacrifice him. Throughout the Old Testament we find many types of Christ. Here, Abraham is a type of God the Father: he is willing to sacrifice his son. Isaac is here a type of Christ: he accepts the will of his father, even if it means death.
And notice that, after God stays the hand of Abraham, He refers to Isaac as Abraham’s “only son.” Clearly, God is not speaking in earthly or physical terms, as Ishmael is also Abraham’s son. He is referring to the line of promise – the spiritual line of salvation. Which is why, after the faith of Abraham (who believed that God would, and in a sense did, resurrect his only son – see Heb 11:17-19) results in obedience, God again reaffirms His promise and His covenant. Through Isaac, Abraham will have offspring as numerous as the stars of heaven and the sand on the seashore – and all the nations of the earth will be blessed through that offspring.
And while we may be tempted to think of this as some kind of test God put Abraham through – like if he was unwilling to sacrifice Isaac God would remove His blessing – this is not the case. God made His promise, and God keeps His promises. Rather, what we see is how the promise works through faith; how the promise is appropriated through faith. In other words, Abraham and Isaac were already part of the promise because of their faith, and God worked through their faith, and provided everything they truly needed. Including, in His glorious providence, a wife for Isaac (vv. 20-23).
In chapter 23 we have the death of Abraham’s beloved wife, Sarah. After bargaining with Ephron before the judges of the city (the official way it was done then so that the contract would be binding), Abraham buys a cave and a field, and buries Sarah. This is the only land Abraham ever legally owns. He and Sarah do not receive the full physical land that God said He would give him (see Gen 13:14-15). But the physical land was not the ultimate promise.
So what we see in this part of the story is two things. First, what we receive in this life is not the goal. Physical possessions are not the real blessing. God is. Second, we do not receive the real blessings because of what we do, but what God does. We do what we do not to earn the blessing, but because of the blessing. Not to earn the promise, but because God has included us in it.
Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God. – Hebrews 12:1–2 (ESV)