Números 21:9

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” (Romanos 8:3), was made sin for us (2 Coríntios 5:21), and thus fulfilled, as He Himself explained to Nicodemus, the type of the brazen serpent (João 3:14). 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” (Números 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 Reis 18:4).