Years have passed since Genesis 12, Abram already has a son but that is it, that son is not the one that God promised him. So he does not really have a descendant yet, but here is God promising as many descendants as all of those stars that he can see and not see up there. After this sequence of events they entered into a covenant, and the circumcision was begun.
Now there you have it, there is the promise that produces the body of people who, in Romans 9, are in a physical sense within the mass of Israelites, and even as Ishmael, Abraham's son, engendered by natural means, was physically with Abraham's family.
Paul's point in referring to the "promise" is that Isaac's birth was wholly God's work. Get that, it was wholly God's work. Sarah could not bear a child, and the child produced by Abraham through Hagar was disqualified as not being the "child of promise." God was showing that any child produced in the way that Abraham produced Ishmael was not going to be the "child of promise." This is why Paul, in Romans 9, calls these people as being named after Isaac the "children of promise."
So what Paul is talking about in Romans 9 is a separated group, a family named by God after Isaac was born to Abraham and Sarah one year after God made the promise. Therefore, some individuals who begin life as part of the mass of Israelites are separated by God to be part of something else that is also called Israel. Thus we begin to perceive the beginning of a Biblical truth regarding how God sees us in this picture.
Ishmael was somewhere between thirteen or fourteen years older than Isaac, and we find in chapter 17 that he was very tenderly loved by Abraham.
For a good while Abraham clung to the hope that Ishmael might be the promised seed. However God had other plans and the sending away of Hagar and Ishmael apparently was the only way God felt that the inheritance could be kept pure.
So Abraham sees that quite a number of years have gone by, by this time, maybe somewhere in the twenties.
Isaac means laughing and it is a reference to Abraham laughing in joy at the prospect of the possibility of Sarah giving birth to the promised son, and of course it is also a witness against them because of Sarah’s laugh of unbelief. And now here she is saying that everyone will laugh with me in joy and in amazement that I should give birth to a child at age 90.
To pick things up here a little bit and speed on, Abraham sired a child. This man whom God picked, who doubted Him somewhat, went on to even offer his son, Isaac, which means "laughter" because God reminded him of his lack of faith. This man, caused to be the father of the faithful, continually grew to where he could even offer his son before God. This man started out in a weak condition. God had faith in Abraham. His faith was justified and the choice that God made was correct.
Incredulity with a little note of joy. Now on the other hand, Sarah laughed too.
Now there we have incredulity again, only with a note of scorn as well. That is why God was down on her and not down on Abraham's laugh.
