And he shall be thy spokesman. God destroys the pretext for his exemption, by assigning to his brother the office of spokesman, and yet does He not put the other in his place; nay, so merciful is the arrangement, that while He yields to His servant's prayer, He yet confers honor upon him in spite of himself. The offices are thus divided -- Moses is to have the authority, Aaron is to be the interpreter. Thus Moses is set before his brother, from no respect to his own dignity; because the grace of God was to shine forth conspicuously in the head no less than in the members; as it is expressed in these words, that "Aaron should be instead of a mouth, and Moses instead of God;" i e., that he was to dictate what Aaron should faithfully report, and to prescribe what he should obediently follow. By this example did God bear witness that the gifts of the Spirit, as well as our vocations, are distributed by Him at His own good pleasure; and that none excels either in honor or in gifts, except according to the measure of His free bounty. But that the first-born is made subject to the younger, and is only appointed to be his spokesman, whereas God might have accomplished by his hand and labor, what he rather chose to perform by Moses; hence let us learn reverently to regard His judgments, because they are incomprehensible to us, and like a deep abyss. "To be instead of God" is the same as to lead or to direct, or to have the chief command; as the Chaldee Paraphrast [1] renders it, to be the chief or master. It is a very weak calumny of the Arians to abuse this and similar passages, in order to refute the proofs of Christ's divinity, because there is a great difference in speaking of one as God simply and absolutely, and with circumstantial additions. For we know that the name of God is attributed to every potentate, improperly indeed, yet not unreasonably; as when the devil himself is called "the god of this world," (2-Corinthians 4:4;) but wherever mention is made of the true Deity, Scripture never profanes that sacred name.

Footnotes

1 - In the Targum of Onkelos, who has employed rv for the 'lhym of the Hebrew. -- W

Instead of a mouth - We may bear in mind Aaron's unbroken habitude of speaking Hebrew and his probable familiarity with Egyptian.
Instead of God - The word "God" is used of persons who represent the Deity, as kings or judges, and it is understood in this sense here: "Thou shalt be to him a master."

He shall be thy spokesman - Literally, He shall speak for thee (or in thy stead) to the people.
He shall be to thee instead of a mouth - He shall convey every message to the people; and thou shalt be to him instead of God - thou shalt deliver to him what I communicate to thee.

And he shall be thy spokesman unto the people: and he shall be, [even] he shall be to thee instead of a mouth, and thou shalt be to him instead of (g) God.
(g) Meaning, as a wise counsellor and full of God's spirit.

And he shall be thy spokesman unto the people,.... And open to them Moses's commission from God, and the end of his mission into Egypt, and to them, and declare what signs had been, and would be done, in confirmation of it:
and he shall be, even he shall be to thee instead of a mouth; or an interpreter, as all the Targums explain it, and so Jarchi; as he was an orator and master of language, he should speak to the people for Moses, and explain his sense and meaning, and put it into plain, proper, easy language, to be understood by the people; and this may be done where a different language is not spoken, but the same in plainer words, in more pertinent expressions, and better pronounced, and this is repeated for the certainty of it:
and thou shall be to him instead of God; Aaron was to stand between Moses and the people, and speak for him; and Moses was to stand between God and Aaron, and in God's stead, and tell him what orders he had received from him, and which he should communicate; and so some Jewish writers (a) interpret it of his being to him instead of a master or teacher, one that received doctrine from the Lord, and instructed him in it, and taught him the mind and will of God: or, as Onkelos paraphrases it; "for a prince", and so Jarchi, a civil magistrate, one that had the power of life and death; the administration of civil affairs belonged to Moses, and Aaron, though the elder brother, was subject to him; and in this sense Moses was a god to him; and so in after times, the judges of Israel, they that sat in Moses's chair, were called gods, Psalm 82:1.
(a) Targum Jonah. Jerus. & Abendana in loc.

Instead of God - To teach and to command him.