"That which is born of the flesh is flesh; and that which is born of the Spirit is spirit" (John 3:6).

When Jesus told Nicodemus he was unfit for the kingdom of God without a second birth, it was saying a lot.  If corruption comes by the first birth, it means that the whole human race is in a fallen condition.  The fact that every individual has a long record of personal sins is the confirmation of this.  Each of us needs new life, a new heart, and a fresh start.

Nicodemus asked a rather obvious question on all our behalf.  "Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?" (John 3:4).  Jesus' answer in the opening quote from verse 6 helps us all understand what a new birth really is and secures us from being deceived by counterfeits.

It is a spiritual birth rather than a natural process.  It is beyond the power of man to bring about of himself, by the resources and faculties he started off with under the first birth.  It is consequently beyond the supposed analysis of Freudian theory, or such investigations, since it is not just a psychological or emotional phenomenon.  It is something supernatural -  in the good sense of the word.
Jesus plainly said that it is accomplished by the action of the Holy Spirit.  This means that it is an operation of God in the life, and that man-made religion built on human agency is incapable of bringing it about.  The Bible speaks of many different works of the Holy Spirit;  But this fundamental matter of the New Birth is simply God's saving power applied to an individual to make a new person in Christ. The Holy Spirit gives a new heart and a new life.
Jesus asked Nicodemus, " Art thou a master of Israel, and knowest not these things?" (verse 10b).  Israel's prophets spoke the same message.  "A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.  And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them" (Ezekiel 36: 26-27).

"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" (John 3:14-15).

The occasion in the life of Moses referred to here by Jesus can be read of in Numbers 21.  Israel had sinned against the Lord and judgment followed.  "And the LORD sent fiery serpents among the people, and they bit the people; and much people of Israel died" (Numbers 21:6).  This is just one of the many specific instances of sin recorded in the Bible so we who would come after could learn the ways of the Lord and the dangers of sin.  Sin brings the deadly bite of the serpent.

When the people under conviction cried, "we have sinned", God provided a way of escape.  "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" (Numbers 21:8).

The serpent of brass mirrored the death sentence Israel experienced by the fiery serpents, but lifted up on a pole it was an offer of life for all that were bitten.  Jesus referred to this to teach Nicodemus of God's provision for eternal salvation soon to take place.  Jesus had to be lifted up on a cross and experience the curse against sin, the death penalty.  Though He had never sinned, the Saviour had to die.  He "must" be lifted up.
The snake-bitten, guilty, dying men of Israel could only have physical life restored by looking at the brass serpent lifted up.  And Nicodemus (or you or I) can only have eternal salvation by a look of faith toward the Lord Jesus Christ in His substitutionary death on the cross (believe).  Nicodemus had first just wanted the faith of the scholar, the seminary student.  "The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him" (John 3:2).  But the faith that saves is the one that looks to God's Son dying on the cross for salvation like the guilty snake-bitten sinners of Numbers 21.