No one has ascended into heaven except he who descended from heaven, the Son of Man. And 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 3:13-15 ESV)
Jesus led Nicodemus deep into the Story of God’s word and reminded him of the perpetual conflict between the Serpent and the Savior. (Numbers 21:4-9) Then Jesus showed Nicodemus that he is the true and better serpent who saves his people from the evil serpent.
Why would Jesus tell this particular story to Nicodemus?
By telling this story Jesus was implying that Nicodemus and his fellow Jewish leaders were like sojourners in the wilderness — just like their forefathers.
And just as the Israelites did not appreciate manna and water in the desert, so Nicodemus and the Jewish Ruling Council (Sanhedrin) did not rightly appreciate the Christ and the Holy Spirit. Therefore, they were under divine judgment.
Finally, by telling this story Jesus was implying that just as God provided temporal salvation for the Israelites from the serpents’ bite through the bronze serpent on a pole, so now God would provide eternal salvation for the world from the ancient serpent through the Son of Man lifted up on the cross.
The Greek word for “lifted up” means raise, lift, or exalt. In the Gospel of John it refers to Jesus being lifted up on the cross, lifted up from the grave, and lifted up into heaven.
So Jesus connected the story of the bronze serpent to his own story in such a way that Nicodemus was enabled to see the up lifted serpent as a sign-post that pointed to the Christ — as a shadow of Jesus who was lifted up to save his people. How?
What does the serpent have to do with Jesus?
First, in Genesis 3 God cursed the serpent and established enmity between his seed and the seed of the woman. God promised that the serpent would bruise the seed of the woman, but he would crush the serpent’s head.
In Exodus 7 the serpent-staff of Aaron the High Priest swallowed up the serpents of the Egyptian magicians.
In Numbers 21 a plague of fiery serpents attacked the Israelites. God commanded Moses to fix a copper serpent on a pole in order to save the people. The snake-bitten people were saved by a serpent fix to a pole.
Finally, in 2 Kings 18:4 the copper serpent which Moses had made was found in the temple. The Israelites idolized and worshiped it and made offerings to it.
What is the connection between these stories, the serpent, sin, and salvation?
Jesus Christ — the Son of Man and Son of God – is:
the seed of the woman who crushed the serpent’s head;
the sympathetic high priest whose cross swallowed up the serpent;
the savior who became a serpent to save us from our sins (he who knew no sin became sin for us that we might become the righteousness of God); 
the righteous king who destroyed the relic of the serpent’s image and delivered his people from idols.
Jesus is the true and better serpent who saves his people from the serpent by becoming a serpent and swallowing up the serpent for his people.
He was lifted up so that whoever trusts in him may be delivered from evil, sin, and death, and may have eternal life.
So, if you would be saved, turn your eyes upon Jesus, look full in his wonderful face, and trust him with all your heart, and you will live.
 Here world refers to Jews, Samaritans, and Greeks. Jesus purchased redemption for the whole world (1 John 2:2) — for a people from every tribe, language, nation, and people (Rev 5:9).
 Galatians 4:4
 Hebrews 2:14; 4:14-16