What man can do the Will of God
Giving commandments to men suggests that a man can do the Will of God exactly as God intended for them to do. But the only one who knows the Will of God is God alone. All others can do is try to do what He wants but misunderstand to some extent because they're not God that they should know exactly what He meant. So the question becomes whether the law of Moses is sufficient and complete enough for others to do the Will of God exactly as He intended? If not, then giving the law implies that God must become a man just to show them what He meant.
If the law of Moses was sufficient and complete in and of itself, then there would be no need for others to come and give more of God's commandments. But even the law of Moses predicts that another prophet will come and declare all that God commands, Deuteronomy 18:14-19.
Thus, giving the law implies that a man will do the Will of God. But the law is insufficient to completely guide men in the Will of God since it predicts that other words will come. So only God can know what His Will is which He orders to be done. And so giving the law implies that God will become a man who will fulfill the intent of the law which was admittedly insufficient since it predicts the coming of other commands.
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