Ambrose of Milan : The Lord also said to the Apostles, opening their mouths, Receive ye the Holy Ghost, whereby He declared that He is the same Who said to Moses, I will open thy mouth, and teach thee what thou shalt say. Wherefore this wisdom, divine, unspeakable, unadulterated and incorruptible, pours her grace into the minds of her saints, and discloses to them knowledge that they may behold her glory. — Letter 65.4 Augustine of Hippo : 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." — QUESTIONS ON Exodus 9