He received circumcision as a sign or seal of the righteousness which he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised and who thus have righteousness reckoned to them,
and likewise the father of the circumcised who are not merely circumcised but also follow the example of the faith which our father Abraham had before he was circumcised.
That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his descendants -- not only to the adherents of the law but also to those who share the faith of Abraham, for he is the father of us all,
as it is written, "I have made you the father of many nations" -- in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist.
He did not weaken in faith when he considered his own body, which was as good as dead because he was about a hundred years old, or when he considered the barrenness of Sarah's womb.