As well as father of the circumcised-to those who are not only circumcised, but who also follow our father Abraham in that faith which he had while still uncircumcised.
This means that it is not the children born in the course of nature who are God's Children, but it is the children born in fulfillment of the Promise who are to be regarded as Abraham's descendants.
It was faith that enabled Noah, after he had received the divine warning about what could not then be foreseen, to build, in reverent obedience, an ark in which to save his family. By his faith he condemned the world, and became possessed of that righteousness which follows upon faith.