Now therefore go, and I will be with your mouth, and teach you what you shall say.
All Commentaries on Exodus 4:12 Go To Exodus 4
Augustine of Hippo
AD 430
It is clear that not only the instruction that comes from his mouth but also its being opened pertains to the will and grace of God. For God does not say, “You open your mouth, and I will instruct you,” but promised both: “I shall open, and I shall instruct.” Elsewhere he says in a psalm, “Open your mouth, and I shall fill it.” There it signifies the will in man to receive what God gives to one who is willing, so that “open your mouth” pertains to the initiative of the will and “I shall fill it” to the grace of God. But here the sense is “I shall both open your mouth and instruct you.”