Galatians 3:6 Even as Abraham believed God, and it was accounted to him for righteousness.
7 Know ye therefore that they which are of faith, the same are the children of Abraham.
Paul here is using Abraham as proof that there has never been any other way of salvation than by grace though faith. Even the Old Testament teaches justification by faith.We should all be righteous because it is the moral condition that is acceptable to God. Anyone who has ever pleased God has done so because they believed. Believing Jews and Gentiles are the true spiritual children of Abraham because they follow his example of faith. We must believe the same thing Abraham believed if we are to truly be considered children of Abraham. The one thing that set Abraham aside from all others was that he believed God. The thing that should separate all believers in Christ from the rest of the world is that we believe Christ.