It is central to Paul’s position that the way of salvation he has just been outlining, the way of salvation by God’s grace, is no new thing. Now he proceeds to show that this was true of Abraham, the great father of the Isreal. God had acted in grace towards Abraham, and Abraham had been justified by faith - that is Paul’s argument. Whereas if Abraham had been accepted on the grounds of his works, Paul’s point that God had always acted in grace would not stand. Abraham is critically important for the listeners to understand that we are justified by faith, and it is important for us that we understand that all that came before Jesus was still part of God’s plan.