Charles Ellicott Commentary Numbers 21:9

"And Moses made a serpent of brass, and set it upon the standard: and it came to pass, that if a serpent had bitten any man, when he looked unto the serpent of brass, he lived." — Numbers 21:9 (ASV)

And Moses made a serpent of brass.— The old serpent was the cause of death, temporal and spiritual. Christ Jesus, in the likeness of sinful flesh (Romans 8:3), was made sin for us (2 Corinthians 5:21), and thus fulfilled, as He Himself explained to Nicodemus, the type of the brazen serpent (John 3:14–15). The meaning of this type, or sign of salvation, is explained in the Book of Wisdom in these words, He that turned himself toward it was not saved by the thing that he saw, but by Thee, that art the Saviour of all (Numbers 16:7). This serpent was preserved by the Israelites, and taken into Canaan, and was ultimately destroyed by King Hezekiah, after it had become an object of idolatrous worship (2 Kings 18:4).