Thou shalt speak - Moses thus retains his position as "mediator;" the word comes to him first, he transmits it to his brother.

I will be with thy mouth, and with his mouth - Ye shall be both, in all things which I appoint you to do in this business, under the continual inspiration of the Most High.

And thou shalt speak unto him, and put words in his mouth,.... Or "things" (z), the matter and substance of what he should say, who being a man of words, an eloquent man, and a good spokesman, would put them into proper language, and express them fluently:

and I will be with thy mouth, and with his mouth, and will teach you what ye shall do; or speak what Moses should say to Aaron, and what Aaron should say to Pharaoh, and to the people of Israel; so that as Aaron was under Moses, and at his direction, they were both dependent on the Lord, and under his direction; and the one, as well as the other, needed his assistance, even Aaron that could speak well. Moses furnished him with matter, he put it into words, and both were instructed and influenced by the Lord what they should say and do.

(z) .

And thou shalt speak unto him, and {f} put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do.

(f) You will instruct him what to say.

4:15 I will be with thy mouth and with his mouth - Even Aaron that could speak well, yet could not speak to purpose, unless God were with his mouth; without the constant aids of divine grace, the best gifts will fail.

4:10-17 Moses continued backward to the work God designed him for; there was much of cowardice, slothfulness, and unbelief in him. We must not judge of men by the readiness of their discourse. A great deal of wisdom and true worth may be with a slow tongue. God sometimes makes choice of those as his messengers, who have the least of the advantages of art or nature, that his grace in them may appear the more glorious. Christ's disciples were no orators, till the Holy Spirit made them such. God condescends to answer the excuse of Moses. Even self-diffidence, when it hinders us from duty, or clogs us in duty, is very displeasing to the Lord. But while we blame Moses for shrinking from this dangerous service, let us ask our own hearts if we are not neglecting duties more easy, and less perilous. The tongue of Aaron, with the head and heart of Moses, would make one completely fit for this errand. God promises, I will be with thy mouth, and with his mouth. Even Aaron, who could speak well, yet could not speak to purpose, unless God gave constant teaching and help; for without the constant aid of Divine grace, the best gifts will fail.