Explanatory Notes on the Whole Bible, by John Wesley, [1754-65], at sacred-texts.com
exo 7:1I have made thee a god to Pharaoh - That is, my representative in this affair, as magistrates are called gods, because they are God's vicegerents. He was authorized to speak and act in God's name, and endued with a divine power, to do that which is above the ordinary course of nature. And Aaron shall be thy prophet - That is, he shall speak from thee to Pharaoh, as prophets do from God to the children of men. Thou shalt as a god inflict and remove the plagues, and Aaron as a prophet shall denounce them.
exo 7:7Moses was fourscore years old - Joseph, who was to be only a servant to Pharaoh, was preferred at thirty years old; but Moses, who was to be a god to Pharaoh, was not so dignified till he was eighty years old. It is fit he should long wait for such an honour, and be long in preparing for such a service.
exo 7:9Say unto Aaron, Take thy rod - This Moses ordinarily held in his hand, and delivered it to Aaron upon occasion, for the execution of his commands.
exo 7:10And Aaron cast his rod down, and it became a serpent - This was proper not only to affect Pharaoh with wonder, but to strike a terror upon him. This first miracle, though it was not a plague, yet amounted to the threatening of a plague; if it made not Pharaoh feel, it made him fear; this is God's method of dealing with sinners he comes upon them gradually.
exo 7:11Moses had been originally instructed in the learning of the Egyptians, and was suspected to have improved in magical arts in his long retirement. The magicians are therefore sent for to vie with him. The two chief of them were Jannes and Jambres. Their rods became serpents; probably by the power of evil angels artfully substituting serpents in the room of the rods, God permitting the delusion to be wrought for wise and holy ends. But the serpent which Aaron's rod was turned into, swallowed up the others, which was sufficient to have convinced Pharaoh on which side the right lay.
exo 7:13And he harden'd Pharaoh's heart - That is, permitted it to be hardened.
exo 7:20The waters that were in the river were turned into blood - This was a plague justly inflicted upon the Egyptians; for Nilus the river of Egypt was their idol; they and their land had so much benefit by that creature, that they served and worshipped it more than the creator. Also they had stained the river with the blood of the Hebrew children, and now God made that river all bloody; thus he gave them blood to drink, for they were worthy, Rev 16:6. See the power of God. Every creature is that to us which he makes it to be, water or blood. See the mutability of all things under the sun, and what changes we may meet with in them. That which is water to day may be blood to morrow; what is always vain may soon become vexatious. And see what mischievous work sin makes! It is sin that turns our waters into blood.
exo 7:22And the magicians did so - By God's permission with their enchantments; and this served Pharaoh for an excuse not to set his heart to this also, (Exo 7:23,) and a poor excuse it was. Could they have turned the river of blood into water again, it had been something; then they had proved their power, and Pharaoh had been obliged to them as his benefactors.
exo 7:25Seven days were fulfilled - Before this plague was removed.