And Moses made a serpent of bronze, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of bronze, he lived.
Read Chapter 21
Augustine of Hippo
To be made whole of a serpent is a great sacrament. What is it to be made whole of a serpent by looking upon a serpent? It is to be made whole of death by believing in one dead. And nevertheless Moses feared and fled. What is it that Moses fled from that serpent? What, brethren, save that which we know to have been done in the gospel? Christ died, and the disciples feared and withdrew from that hope wherein they had been.
The serpent struck Adam in paradise and killed him. [It also struck] Israel in the camp and annihilated them. “Just as Moses lifted up the serpent in the desert, the Son of Man will be lifted up.” Just as those who looked with bodily eyes at the sign which Moses fastened on the cross lived bodily, so too those who look with spiritual eyes at the body of the Messiah nailed and suspended on the cross and believe in him will live [spiritually]. Thus it was revealed through this brazen [serpent], which by nature cannot suffer, that he who was to suffer on the cross is one who by nature cannot die. Commentary on Tatian’s Diatessaron
A brazen serpent. This was a figure of Christ crucified, and of the efficacy of a lively faith in him, against the bites of the hellish serpent, John iii. 14. (Challoner) (St. Ambrose; Apol. i. 3.) As the old serpent infected the whole human race, Jesus Christ gives life to those that look at him with entire confidence. (Theodoret, q. 38.) The brazen serpent was destitute of poison, though it resembled a most noxious animal; so Jesus Christ assumed our nature, yet without sin. (Calmet)
That brazen serpent was hung up as a remedy for the biting serpents, not as a type of him that suffered for us but as a contrast. It saved those that looked upon it, not because they believed it to live but because it was killed, and killed with it were the powers that were subject to it, being destroyed as it deserved. And what is the fitting epitaph for it from us? “O death, where is your sting? O grave, where is your victory?” You are overthrown by the cross. You are slain by him who is the giver of life. You are without breath, dead, without motion, even though you keep the form of a serpent lifted up high on a pole.
Tell me, did not God, through Moses, forbid the making of an image or likeness of anything in the heavens or on earth? Yet didn’t he himself have Moses construct the brazen serpent in the desert? Moses set it up as a sign by which those who had been bitten by the serpents were healed. In doing so, was Moses not free of any sin? By this, as I stated above, God through Moses announced a mystery by which he proclaimed that he would break the power of the serpent, who prompted the sin of Adam. He promises that he would deliver from the bites of the serpent (that is, evil actions, idolatries and other sins) all those who believe in him who was to be put to death by this sign, namely, the cross.