In Numbers 21:4-8 it is written that the Israelites grew impatient on the journey and spoke against God and against Moses saying: “Why have you led us up out of Egypt to die in the wilderness? There is no bread or water, and we detest this wretched food (Manna)!” In regards to this, the LORD sent venomous snakes among the people, and many of the Israelites were bitten and died.
Then the people came to Moses and said, “We have sinned by speaking against the LORD and against you. Intercede with the LORD to take the snakes away from us.” So Moses interceded for the people. Then the LORD said to Moses, “Make a fiery serpent and mount it on a pole. When anyone who is bitten looks at it, he will live.” So Moses made a bronze snake and mounted it on a pole. If anyone who was bitten looked at the bronze snake, he would live.
When Nicodemus came secretly by night speaking to Jesus, He said to Nicodemus:
"Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, that everyone who believes in Him may have eternal life."
I have always loved this comparison. The Israelites who were bitten by the poisonous snakes would have surely died. But as soon as they obeyed the word of the Lord and looked to the bronze serpent that was lifted up, the poison, though coursing through their veins, had no affect on them. Looking at the serpent was an act of faith. In the same way, though we have sin which leads to certain death, when we look to Jesus who was lifted up (crucified), our sin which is comparable to the poison, will in the end, have no affect on the believer. Unfortunately, for those who do not look to Jesus, the results of sin will take its deadly course.