What is New Birth?

John 3:4-7 4 Nicodemus said to Him, "How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?" 5 Jesus answered, "Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. 6 "That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. 7 "Do not marvel that I said to you, 'You must be born again.'

New Birth is regeneration, it is the Baptism of the Holy Spirit the circumcision of the heart.
It is the work of God in which He makes the dead sinner a living saint and grants him to enter into the kingdom of God.
Nicodemus asks How can there be 2 births? Jesus answers that there must be 2, a natural (water) and a supernatural (Spirit) to enter the kingdom of God.

Some have tried to attack the doctrine of Regeneration by teaching that Jesus is speaking of 3 births in this passage. Natural, which is not mentioned but implied, a birth through the word (water) and a supernatural birth (spirit). This view is another gospel as it teaches that natural men can come to the kingdom of God in steps.

In the first step spiritually dead men hear the gospel and of their own free will believe causing God to begin the second step and respond to that faith with a second blessing or spirit birth.

This view turns the new birth into new births and the grace of God in salvation in to the work of men.