"And the Lord sent fiery serpents among the people, and they bit the people; and much people of Israel died. Therefore the people came to Moses, and said, We have sinned, for we have spoken against the Lord, and against thee; pray unto the Lord, that he take away the serpents from us. And Moses prayed for the people. And the Lord said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live. 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." (Numbers 21:6-9, emphasis added)
The unfaithful Israelites were dying from getting bitten by poisonous snakes. As a result, the Jewish people needed an antidote to ensure their survival after envenomation by these creatures. They were God's curse to punish His chosen people for sin and rebellion. In response to the people's plea for clemency, God instructed the Israelites to simply look at the bronze serpent, which was created by Moses. Those who placed their trust in the Lord by looking at it miraculously got rescued from the sentence of death. We can infer from this historical event the spiritually bankrupt nature of man. Jesus Christ Himself is the typological fulfillment of the bronze serpent:
"And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: That whosoever believeth in him should not perish, but have eternal life. For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved. He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God." (John 3:14-18, emphasis added)
Everybody has been spiritually poisoned by sin. This Old Testament incident of people getting spared from physical death is a typological illustration of Jesus Christ's power to save us from spiritual death. Those who turn to Christ by trusting in His redemptive work are saved from eternal condemnation. Sinners are cured of their spiritual illness by the Great Physician, Jesus Christ. The Jews were not saved by good works, but by simply placing their faith in God. The atonement of Christ is applied to all who come to Him by faith through the gospel.