Paul is bringing out the fact in using Abraham as an example that we are saved through faith alone not works of any kind. He says that Abraham believed God and it was accounted to him for righteousness. Our sin debt is too heavy to pay with the best of our works. David affirms this too in Psalm 32.
When God singled out Abraham and his family as a people of faith, He then gave him and the future men of his family the sign called circumcision. Since then all of the men of Israel were to be circumcised.
Now we have a question about us Gentiles who have faith but not circumcised Israelites. We are called children of Abraham because of our faith even thought we don't have circumcision (men) as an outward sign in the flesh. In other Bible verses Paul calls it a circumcised heart.
So I can call myself, because of faith not works in Jesus Christ, a son of Abraham. Even though he did not personally know about Jesus Christ, he was a man of faith. Are you a person who has that kind of faith?