Abraham let himself be circumcised to show that he had been accepted because of his faith even before he was circumcised. This makes Abraham the father of all who are acceptable to God because of their faith, even though they are not circumcised.
Everything depends on having faith in God, so that God's promise is assured by his great kindness. This promise isn't only for Abraham's descendants who have the Law. It is for all who are Abraham's descendants because they have faith, just as he did. Abraham is the ancestor of us all.
And because of what Jesus Christ has done, the blessing that was promised to Abraham was taken to the Gentiles. This happened so that by faith we would be given the promised Holy Spirit.