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.
What God did here was add circumcision as a requirement and as the sign that those bearing this mark had made the covenant at the same time with Abraham. (I believe that this term "selfsame day" appears something like fourteen times in the Bible.)
