John Calvin Commentary Exodus 7:8

"And Jehovah spake unto Moses and unto Aaron, saying," — Exodus 7:8 (ASV)

And the Lord spake. It is no wonder that Moses often repeats the same thing, because he wrote for people with unrefined and dull minds. However, it is fitting for us, so that we are not put off by his simple and popular style, to diligently examine how little we are inclined to be perceptive and earnest in our consideration of God's works.

Undoubtedly, what is related here is what we have already heard concerning the change of the rod into a serpent. The exception is that he now tells us that the miracle, previously performed in the wilderness of Midian and later in Egypt in the sight of the people, was also performed once more before Pharaoh.

Moreover, we gather from this that at Pharaoh's request, God's servants had proved and testified to their calling. Therefore, his stubbornness was all the less excusable, since he despised God's power so clearly displayed.

For this is typical of unbelievers: to demand proofs of God’s power, which they may still discredit—not because they openly scorn God, but because their secret impiety urges them to seek evasions.

The message is disagreeable and deeply annoying to the proud king. Because he does not dare to refuse God directly, he invents a plausible pretext for his refusal by asking for a miracle. When this is performed, he seeks even deeper hiding places, as we will very soon perceive.

Therefore, since it was certain that he would not willingly obey the divine command and would not yield before being miraculously convinced, God provides His servants with a remarkable and sure testimony of His power.

Furthermore, the change of the crook, or shepherd’s staff, into a serpent had this purpose: that the humble and rustic appearance of Moses should not be despised.

For (since kings are accustomed to exalt themselves greatly) Pharaoh might have laughed at the boldness of Moses and Aaron. They, seemingly forgetful of their own condition, put themselves in conflict with the entire power of Egypt. However, Pharaoh knew—although they were not to be feared for their splendid appearance and had nothing magnificent about them—that they were still not lacking sure and strong help when he saw the serpent emerge from the rod.

In short, God bore witness that His power is hidden beneath the weakness of His servants, so that at all times He might make formidable to the greatest monarchs those who are otherwise like earthen vessels.

It is not clear to me why Aaron was commanded to cast down the rod rather than Moses, unless perhaps because God intentionally sought to humble the pride of the arrogant king. He did not condescend to exert His power by the hand of His higher-ranking servant but only employed the lower-ranking one.

Therefore, with reference to this ministry, the rod of God and of Moses is now called the rod of Aaron. In the same way, Paul boasts of his gospel, the office of preaching which he knew was committed to him (Romans 16:25 and 2 Timothy 2:8).