Jesus answered him, "Most assuredly, I tell you, unless one is born anew,{The word translated "anew" here and in John 3:7 (anothen) also means "again" and "from above".} he can't see the Kingdom of God."
The doctrine of the new birth will be discussed under  John 3:5, where Jesus more fully described it.  Here the emphasis is upon the absolute necessity of it.  It is not merely true that one cannot enter God's kingdom without the new birth; he cannot even see it!  The requirement here stated by Jesus was actually a demand that Nicodemus forsake all reliance upon the law of Moses, and upon the elaborate ritual and traditionalism of the Pharisees, and enter upon a totally new way of life.  It was a shocking requirement; and the evidence is that Nicodemus, at that point in time, was not able to accept it.