As Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life. John 3:14, 15, ESV.
Here Jesus takes us back to a story recorded in Numbers 21:4-9. In it the children of Israel were up to their favorite "churchly" pastime--grumbling and complaining. They regretted ever having left Egypt.
In punishment for their lack of faith in the face of His miraculous leading, God sent them a plague of deadly fiery serpents. Repenting, the people cried for mercy. In response, the Lord instructed Moses to make an image of one of the serpents and to hold it up in the midst of the camp. All who followed God's instruction in faith and looked upon the serpent would be healed.
The Jewish people loved that story and were fond of retelling it. The major problem with it was that God had also told them never to make an idol or to worship one. But the Lord had commanded the manufacture of this one, in spite of the fact that the serpent had been a symbol of Satan since Eden. The Jews apparently stored the bronze serpent itself in the tabernacle and later the Temple as a sacred object until King Hezekiah, after discovering that people were worshipping it, had it broken in pieces (2 Kings 18:4). About the time of Jesus one Jewish writer found it necessary to point out that it wasn't the bronze serpent that had healed the people, but God (Wisdom of Solomon 16:7)
At any rate, Jesus used that rather peculiar Jewish story to make an important point in closing off His discussion with Nicodemus: Just "as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life."
Like other portions of the discussion with Nicodemus, we find a mystery here. After all, how can we possibly compare the crucifixion of Jesus to a snake on a pole? Isn't the snake in biblical history the problem rather than the solution?
While we might not understand all the ins and outs of what Jesus said, His meaning is absolutely clear: that a faith response on our part conditions spiritual healing, the new birth, and the working of the Spirit in our hearts and lives, just as it did among the ancient Israelites in the desert.
Individuals are not born saved for eternal life. It comes through looking at Jesus upon the cross in an attitude of profound faith in the power of God.