After explaining that Abraham understood that God was promising the he and his offspring would inherit the earth and not just the land in the Middle East, Paul then continues to explain why it was an act of faith. Abraham knew that he was dead in terms of being able to have children. He knew that Sarah had never had children. But when God promised that they would have a son, Abraham believed Him even when it was against how the world said everything worked. As it says in Hebrews 11, he believed that the invisible caused the visible and not the other way around. So through faith, He believed that God would do what He promised. God recorded that it was his faith that caused Abraham to be accounted righteous, so that we would understand that righteousness is not by works, but by faith.