In what sense is Abraham the father of all believers?
Abraham is the father of all believers in the spiritual realm, not by physical descent or law. Scripture calls him the father of us all because he was the first to walk by faith without law and without circumcision, coming out of idolatry by faith. Believers likewise come by that same faith.
Les explains that the promise is sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all. He clarifies that this is true in the spiritual realm, because Abraham was the first who walked by faith without law and without circumcision. He came out of abject idolatry and walked by faith, and Les says that is where we have to come from. This connects with grace, which must be kept as God's unmerited favor, not something earned by human effort.
Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,