He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the ancestor of all who believe without being circumcised and who thus have righteousness reckoned to them,
and likewise the ancestor of the circumcised who are not only circumcised but who also follow the example of the faith that our ancestor Abraham had before he was circumcised.
For this reason 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 all of us,
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.
Hoping against hope, he believed that he would become "the father of many nations," according to what was said, "So numerous shall your descendants be."
He did not weaken in faith when he considered his own body, which was already as good as dead (for he was about a hundred years old), or when he considered the barrenness of Sarah's womb.