And Moses and Aaron went in unto Pharaoh, and they did so as the LORD had commanded: and Aaron cast down his rod before Pharaoh, and before his servants, and it became a serpent.
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Ambrose of Milan
He cast down his rod, and it became a serpent which devoured the serpents of Egypt. This signified that the Word should become flesh to destroy the poison of the dread serpent by the forgiveness and pardon of sins. For the rod stands for the Word that is true, royal, filled with power and glorious in ruling. As the rod became a serpent, so he who was the Son of God begotten of the Father became the Son of man born of a woman. Like the serpent, he was lifted up on the cross, poured his healing medicine on the wounds of humanity. Wherefore the Lord himself says, “As Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up.” .
For by the serpent is to be understood death, which was brought about by the serpent in paradise, according to the manner of speech which attributes the effect to the cause. Therefore the rod was turned into a serpent, and the whole Christ, together with his body which is the church, into the resurrection, that will take place at the end of time. This is signified by the tail of the serpent which Moses held, in order that it might be turned again into a rod. But the serpents of the magicians are like those who are dead in the world, for, unless by believing in Christ they have been as it were swallowed up and entered into his body, they will not be able to rise in him.
The staff is a sign of the cross. It caused all the plagues when it swallowed the snakes, just as [the cross] would destroy all idols. With [the staff], [Moses] divided the sea and drowned the Egyptians. That prefigured the destruction of the Canaanites.