It says this because there is no difference between those who are Jews and those who are not. The same Lord is the Lord of all people. And he richly blesses everyone who looks to him for help.
Abraham was circumcised later to show that God accepted him. His circumcision was proof that he was right with God through faith before he was circumcised. So Abraham is the father of all those who believe but are not circumcised. They believe and are accepted as people who are right with God.