And the anger of the LORD was kindled against Moses, and he said,
Is not Aaron the Levite your brother? I know that he can speak well. And also, behold, he comes forth to meet you: and when he sees you, he will be glad in his heart.
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Augustine of Hippo
How can the anger of God be understood, since God is not gripped by any irrational disturbance, as man is? Where Scripture says something like this, we should have a consistent explanation to avoid repeating the same account too often. But one can rightly ask why God says here that he is angry with Moses about his brother Aaron, because he would speak to the people for Moses. For it means that God had not given Moses the fullest ability that he was going to give, because he was diffident. God wished the deed to be carried out by two men. He could also have done it through one, if that man had believed. But all these words, when they are considered more diligently, do not mean that the Lord in his anger had handed over Aaron for punishment. For he says this: “Behold, is your brother Aaron not a Levite? I know that when he speaks, he will speak eloquently.” These words show that God rather reproached Moses, who feared to go because he was less suitable, since he had a brother through whom he could say to the people what he wanted, because Moses himself had a weak voice and a slow tongue. Still, he should have put all his hope in God. Then he says the same things that he had promised shortly before and afterward grown angry. For he had said, “I shall open your mouth and instruct you.” But now he says, “I shall open your mouth and his mouth, and I shall teach you what to do.” But since he added, “And he will speak for you to the people,” the opening of the mouth seems to be provided, because Moses says he is slow of tongue. But the Lord did not will to supply [vocal strength] for the weakness of his voice but added the help of his brother Aaron. Moses could use Aaron’s voice, which was sufficient to teach the people. So when he says, “and you will put my words in his mouth,” he shows that he was going to provide him with words. For if he were only given things to hear for the people, God would have said, “into his ears.” Then it says a little later, “and he will speak for you to the people,” he shows clearly enough that the leading role was for Moses, the subordinate role for Aaron. What he says thereafter, “You will be to him as God,” perhaps this great mystery is to be examined closely. The figure suggests that Moses was the mediator between God and Aaron, and Aaron the mediator between Moses and the people.