Why can't God use someone who is self-prepared or prepared by the world?
God cannot use someone self-prepared or prepared by the world because such preparation depends on human polish, charisma, power, and worldly wisdom rather than God's presence. Moses had Egypt's advantages, yet Israel rejected him and God was not in that effort. God instead equips whom He calls, promising to be with their mouth and teach them what to say.
Les explains that Moses, after forty years in Pharaoh's palace, had the polish, charisma, and power of Egypt, along with all the wisdom of Egypt. Yet when he first tried to deliver Israel, they rejected him. God was not in that effort, and He could not use Moses that way, because God cannot use someone who is self-prepared or prepared by the world. Later, in Exodus 4, Moses protests that he is not eloquent and is slow of speech. The Lord answers by asking who made man's mouth, who makes the dumb, deaf, seeing, or blind—Have not I the Lord?—and promises, “I will be with thy mouth, and teach thee what thou shalt say.” God had already given Moses signs—the rod becoming a serpent and his hand becoming leprous and then whole—to prove He meant what He said.