Why could God not pour out mercy without grace?
God could not pour out mercy without grace because mercy is not deserved by mankind. Grace is God's unmerited favor that makes the pouring out of His mercy possible. Without that grace, there would be no basis for God to show mercy rather than wrath and judgment.
Les explains that grace is that attribute of God, that unmerited favor, which makes the pouring out of His mercy possible. If it were not for the grace of God, He could never have poured out mercy on mankind, because we do not deserve it. But since grace is unmerited favor, God was able to pour out mercy. Les says he will be teaching that it is the grace of God that makes it possible for Him to show mercy rather than wrath and judgment.