Therefore, the promise is based on faith, so that it may be a matter of grace and may be guaranteed for all of Abraham's descendants not only for those who were given the Law, but also for those who share the faith of Abraham, who is the father of us all.

Now does this blessedness come to the circumcised alone, or also to the uncircumcised? For we say, "Abraham's faith was credited to him as righteousness."

and not all of Abraham's descendants are his true descendants. On the contrary, "It is through Isaac that descendants will be named for you."