9 And Moses made a serpent of brass, 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 brass, he lived.
For tell me, was it not God who commanded by Moses that no image or likeness of anything which was in heaven above or which was on the earth should be made, and yet who caused the brazen serpent to be made by Moses in the wilderness, and set it up for a sign by which those bitten by serpents were saved? Yet is He free from unrighteousness. For by this, as I previously remarked, He proclaimed the mystery, by which He declared that He would break the power of the serpent which occasioned the transgression of Adam, and [would bring] to them that believe on Him [who was foreshadowed] by this sign, i.e., Him who was to be crucified, salvation from the fangs of the serpent, which are wicked deeds, idolatries, and other unrighteous acts.
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.
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.
Many things were done in figure which were done in former times. For when he said that the fathers who were bitten by serpents in the desert could not be healed in any other way except that Moses hung up a brazen serpent, and when this was seen, those deadly bites and injurious effects of the poison were cured, he added: But these things were done in figure to instruct us (1 Corinthians 10:6). In the image, a bronze serpent was placed on a cross; because the true one to be crucified was announced to the human race, who would empty the venom of the devilish serpent, cursed in its image, but in truth, would erase all curses of the world.
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.
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 3. 14.
[AD 165] Justin Martyr on Numbers 21:9