John Calvin Commentary Exodus 4:11

"And Jehovah said unto him, Who hath made man`s mouth? Or who maketh [a man] dumb, or deaf, or seeing, or blind? Is it not I, Jehovah?" — Exodus 4:11 (ASV)

Who has made man’s mouth? Here the cause is expressed why the hesitation of Moses was worthy of rebuke: namely, because, held back by his own weakness, he did not look to God, who, being beyond the need for any human aid, easily accomplishes whatever He has decreed, and subduing all the obstacles that terrify people, obtains assistance from any quarter according to His will.

Moses offers his stammering as a reason for holding back; God replies that it is He alone who governs the tongue that He has created. Therefore, if some are tongueless or dumb, and some quick and eloquent in speech, the difference is entirely due to His good pleasure. Hence it follows that all nature (as it is called) is subject to His government, so that He easily provides means for things that do not yet exist and, on the other hand, removes far out of the way whatever obstacles stand in the way, and even compels them into obedience.

But He not only asserts His right and power of government in the general course of nature, but teaches that it is by His special grace alone that some exceed others in eloquence. And not only that, but it is in His power to make wonderful changes, so as to strike the most eloquent dumb, and to enable the tongue of the dumb to speak. Experience also shows this: sometimes those who excel in readiness of speech are at a loss for words; and, on the contrary, the stammering and slow of speech plead a particular case with admirable dexterity, although they may lack this ability in every other situation.

Since, then, it is in God’s power to bind or to loosen people's tongues at any moment, it was wrong for Moses to hesitate, as if surprised, because he did not possess natural freedom of speech—as if it were not possible for the Author of nature to remedy this disadvantage.

But while it is good to magnify the immense power of God in removing all the hindrances that oppose us, we must also beware of relying on it indiscriminately, as though it were subject to our fancies. For we see people, while they too boldly undertake whatever their own lusts suggest, shielding themselves with the thought that all means and events are in God's hands, so that nothing may stand in the way of their impetuosity.

But the power of God is shamefully profaned by this rashness; and therefore, this truth is not properly applied to its legitimate purpose unless a calling and command clearly call us forward.

We must, then, mark the connection: Go where I will send you. Am I not Jehovah, who gives people speech, and sight, and hearing? The point of which is that Moses, confidently trusting in the bounty of God, should devote himself earnestly to his work.