Paul uses the example of Abraham to illustrate that being considered righteous before God is based on faith and not keeping the Torah. Most likely his opposers used Abraham’s circumcision to insist that the Gentiles should become circumcised to become accepted as part of the people of God. Paul never mentions circumcision but quotes Gen. 15:6 to prove that righteousness comes through faith. Implicit in his argument is that it is either faith or keeping the Torah that makes us righteous. It cannot be both. And he argues it is faith and only faith.  

Paul’s use of Abraham also could be understood that Abraham is a type of Christ. As the father of faith, he has many “sons.” They are his descendants because of their faith. We are sons of God by faith. Abraham’s faithfulness (even if it was sometimes questionable) is also a type of the faithfulness of Christ. The fact that the nations are blessed with Abraham is also a type of Christ becoming a blessing to the whole world. 

Abraham had many failures, yet he is still considered the father of faith. God did not discard him because of his failures. This should be a great encouragement to us. Remember, we live by the faithfulness of the Son of God who loved us and gave himself for us, not by our own merit.