Albert Barnes Commentary Numbers 21:8

"And Jehovah said unto Moses, Make thee a fiery serpent, and set it upon a standard: and it shall come to pass, that every one that is bitten, when he seeth it, shall live." — Numbers 21:8 (ASV)

Make you a fiery serpent - that is, a serpent resembling in appearance the reptiles which attacked the people. The resemblance was of the essence of the symbolism (Compare to 1 Samuel 6:5). As the brass serpent represented the instrument of their chastisement, so looking to it at God’s word denoted acknowledgment of their sin, longing for deliverance from its penalty, and faith in the means appointed by God for healing.

In the serpent of brass, harmless itself, but made in the image of the creature that is accursed above others (Genesis 3:14), the Christian fathers rightly see a figure of Him (John 3:14–15) who though holy, harmless, undefiled, separate from sinners (Hebrews 7:26), was yet made sin (2 Corinthians 5:21), and made a curse for us (Galatians 3:13). And the eye of faith fixed on Him beholds the manifestation at once of the deserts of sin, of its punishment imminent and deprecated, and of the method of its remission devised by God Himself.