In Romans 4, Paul presents the example of Abraham to explain what it means to be justified by faith. When you learn the details of Abraham’s life described in Genesis 11-25, it becomes very clear why Paul says that Abraham was not justified by his good works. Rather, because he believed God, because of his faith, God imputed his own righteousness to Abraham and counted Abraham as righteous. As Hebrews 11:1-2 makes clear, Abraham was confident in what he hoped for and assured about what he did not see because he was confident that God would fulfill what he had promised.
