After showing that Abraham was saved by faith and not by works, Paul argues the foolishness of thinking that we could be saved by works. To be saved by works means that God owes us something. That are works have created a debt of wages from God to a man. When God has given us all things including our very life itself, the idea that God could have a debt to us is completely contrary to the nature of God. He then goes on and says that David understood this as he quotes from Psalm 32.