Numbers 21:9

And Moses made a serpent of brass, as much in form and appearance like the fiery serpents as possible, 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, that is, if he looked at it with the faith which the occasion required, he lived. It was because the Israelites, with their sin, tempted Christ, that they were destroyed by the serpents, 1 Corinthians 10:9. And, on the other hand, because they had faith in the promises of God, they were healed. Note that the figure made by Moses was a type of Christ, John 3:15. God sent His Son in the form of our sinful flesh, but without sin. And Christ, the Holy One of God, was lifted up on the cross to expiate the sin of all mankind, which lay upon Him. No matter who it is among sinful men, if he but looks upon the crucified Christ in faith, he will not perish, but have everlasting life.