What kind of people is God unable to use?
God is unable to use people who are self-prepared or have been prepared by the world. Such individuals rely on their own polish, charisma, and power rather than on God's enabling. Instead, God works through those who acknowledge their inadequacy and depend on Him, as illustrated by Moses' own confession of being slow of speech.
Les explains that Moses had spent forty years in Pharaoh's palace, possessing all the wisdom of Egypt, polish, charisma, and power. Yet when he tried to deliver Israel in his own strength, they rejected him, and God could not use him that way because God cannot use somebody who is self-prepared or prepared by the world. Later, when Moses protested his lack of eloquence, God reminded him that He is the one who made man's mouth and would be with his mouth and teach him what to say. The signs of the rod and the leprous hand confirmed God's intent to lead Moses.