Continuing his argument concerning justification by faith and using Abraham as his example, Paul turns to the subject of circumcision. Is this necessary for salvation or not?
We see that Abraham was justified by God before he was circumcised, so clearly the circumcision was not what saved but was simply a seal of the righteousness that was reckoned to Abraham and that did save him. Therefore Abraham is a father to those who are not circumcised yet have the faith that he had. He is also the father of those who are circumcised and have the reality that the sign pointed to - the righteousness that comes through faith.
We see that we can add nothing to faith in Christ for our salvation - not circumcision, baptism, church membership, attendance or obedience. We must look to Him alone or we will have fallen away from the principle of salvation by grace alone through faith alone.