In the same way that Moses lifted the serpent in the desert so people could have something to see and then believe, it is necessary for the Son of Man to be lifted up— and everyone who looks up to him, trusting and expectant, will gain a real life, eternal life. John 3:14-15.
 

A man called Nicodemus came to Jesus with a major question.
There was a man of the Pharisee sect, Nicodemus, a prominent leader among the Jews.
Late one night he visited Jesus and said, "Rabbi, we all know you're a teacher straight from God.
No one could do all the God-pointing, God-revealing acts you do if God weren't in on it."
Jesus said, "You're absolutely right. Take it from me: Unless a person is born from above, it's not possible to see what I'm pointing to—to God's kingdom." John 3:1-3.
 

Jesus' reply clearly revealed that it is absolutely necessary to be born again, or saved! The man was a religious leader, but lacked an intimate and personal relationship with God. The only way... I repeat... the only way to get to Heaven is through Jesus Christ; not a church, denomination, creed, family lineage, good deeds and the like. See Acts 4:12; Ephesians 2:8-9.
The conversation then turned to a prophetic declaration about the cross. Salvation or Eternal Life comes by believing In Him... not About Him and Jesus talked about a snake!

To what was Jesus referring?

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 21:8-9.
 

God's people had been living in slavery for 400 years. The seed of Abraham had been taken captive in a foreign land (Egypt) and God delivered them by supernatural signs and wonders.
There were ten plagues in Egypt, all directed at the centre of the Egyptians'  idolatrous worship of false gods.
After the first Passover, they were set free from the land and its despotic ruler, but were not free within from their habits, manners and customs. They left Egypt free from slavery indeed, but took Egypt with them in their hearts.

A new life:

After delivering the people from bondage, they were still in the desert wilderness of Egypt.

It is very possible to be set free from a prison cell, but still remain bound and sadly, far too many believers are like this.
With nowhere to go, their backs were against t he wall so to speak and God opened up the Red Sea to that they could cross over to the other side. Sometimes we need to pass over... to leave the past behind, before we can enter our own Promised Land.

Thir journey could have lasted a couple of weeks, but it took them 40 years!

The people were in the wilderness because they refused to meet God's requirements, but God still loved and cared for them, providing food and water every day. When that generation passed away, ony two people, Joshua and Caleb enterd the Promised Land.