9 Is this blessing then only for the circumcised, or also for the uncircumcised? For we say that faith was counted to Abraham as righteousness. 10 How then was it counted to him? Was it before or after he had been circumcised? It was not after, but before he was circumcised. 11 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 father of all who believe without being circumcised, so that righteousness would be counted to them as well, 12 and to make him the father of the circumcised who are not merely circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised.
When God called Abraham in Genesis 12 he told him that he would bless him and make him a blessing. That promise was fulfilled in Christ as the greatest blessing anyone could receive because through Christ we obtain our salvation.
So in this passage Paul explains that Abraham is the spiritual father of both the Jews and the Gentiles and it all has to do with timing. If Abraham is going to be a blessing to the Gentiles (the uncircumcised) then his faith in God that brought his justification must have been before the seal of the promise God gave him in circumcision, which is exactly what happened. Go back and read his story in Genesis and you will discover that God counted him righteous in chapter 15, but it was not until chapter 17 that he gave him the seal of circumcision. Because he had faith before circumcision and his faith did not come as a result of circumcision then he is able to be the spiritual father of the uncircumcised and the circumcised.
But the story does not end there. There is a direct connection of circumcision to our baptism. True baptism, just like true circumcision, is performed by God in the heart. The outward symbol of water does nothing to affect our hearts, but we are baptized because our hearts have been changed from a heart of stone to a heart of flesh. We are baptized because we have been regenerated by the work of the Spirt and saved by his graciously applying the cleansing blood of Christ to our lives. We are baptized because God has already baptized us in his Spirit. So then our water baptism becomes a sign and a seal to us of what God has already done. We can then, with confidence, look back to our baptism knowing that God’s grace has been given to us and confirmed in us.