That is why all turns upon faith; it is to make the promise a matter of favour, to make it secure for all the offspring, not simply for those who are adherents of the Law but also for those who share the faith of Abraham ??of Abraham who is the father of us all

they are not all children of Abraham because they are descended from Abraham. No, it is through Isaac that your offspring shall be reckoned ??8 meaning that instead of God's children being the children born to him by natural descent, it is the children of the Promise who are reckoned as his true offspring.