In the NT age the Jewish people wanted to boast that they were desendants of Abraham.  They debated who was and who was not.  Paul saw the faith of Abraham was the mark of the sons of Abraham.  James saw that the works of Abraham were a key to being of Abrahamic stock.  It seemed that father Abraham had a rugged obedience to believe that God could raise Isaac from the dead.  He could look at the uncountable stars to be like his descendants, even before he had a child.  He could see the hills and valleys eventually being filled with them.  He also messed up from time to time in getting ahead of God and lying to protect himself.  Which says, God can make imperfect folks children of Abraham too.