He was circumcised as a sign of the covenant God had made with him. It showed that his faith had made him right with God before he was circumcised. So Abraham is the father of all believers who have not been circumcised. God accepts their faith. So their faith makes them right with him.
The promise is based on God’s grace. The promise comes by faith. All of Abraham’s children will certainly receive the promise. And it is not only for those who are ruled by the law. Those who have the same faith that Abraham had are also included. He is the father of us all.
They were written also for us. We believe in the God who raised Jesus our Lord from the dead. So God will accept our faith and make us right with himself.
Christ Jesus set us free so that the blessing given to Abraham would come to the Gentiles through Christ. He did it so that we might receive the promise of the Holy Spirit. The promised Spirit comes by believing in Christ.