John 3 (NIV)
1Now there was a man of the Pharisees named Nicodemus, a member of the Jewish ruling council. 2He came to Jesus at night and said, "Rabbi, we know you are a teacher who has come from God. For no one could perform the miraculous signs you are doing if God were not with him." 3In reply Jesus declared, "I tell you the truth, no one can see the kingdom of God unless he is born again." 4"How can a man be born when he is old?" Nicodemus asked. "Surely he cannot enter a second time into his mother's womb to be born!" 5Jesus answered, "I tell you the truth, no one can enter the kingdom of God unless he is born of water and the Spirit. 6Flesh gives birth to flesh, but the Spirit gives birth to spirit. 7You should not be surprised at My saying, 'You must be born again.' 8The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit." 9"How can this be?" Nicodemus asked. 10"You are Israel's teacher," said Jesus, "and do you not understand these things? 11I tell you the truth, We speak of what We know, and We testify to what We have seen, but still you people do not accept Our testimony. 12I have spoken to you of earthly things and you do not believe; how then will you believe if I speak of heavenly things? 13No one has ever gone into heaven except the One who came from heaven - the Son of Man. 14Just as Moses lifted up the snake in the desert, so the Son of Man must be lifted up, 15that everyone who believes in Him may have eternal life. 16"For God so loved the world that he gave his One and Only Son, that whoever believes in Him shall not perish but have eternal life. 17For God did not send His Son into the world to condemn the world, but to save the world through Him. 18Whoever believes in Him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's One and Only Son. 19This is the verdict: Light has come into the world, but men loved darkness instead of Light because their deeds were evil. 20Everyone who does evil hates the Light, and will not come into the Light for fear that his deeds will be exposed. 21But whoever lives by the truth comes into the Light, so that it may be seen plainly that what he has done has been done through God."
"You must be born again"
Jesus makes a very clear statement in the above passage. You cannot enter the kingdom of God - you can't even see the kingdom of God - unless you are born again. It amazes me to hear people say, "I'm a Christian, but I'm not one of those 'born again' fanatics!" They say this proudly as though being born again is somehow beneath them.
Why is it so difficult for those who are perishing to accept the fact that they must be born again? It may be because they don't understand the concept. The idea is really quite simple. God is Spirit (John 4:24) so you have to approach Him spiritually. And to approach God spiritually, you must be born of His Spirit - you must be born again. That spirit which God originally created and placed in each and every person to allow each and every person to know Him - that spirit which "surely died" (Genesis 2:17) in each and every person with original sin - must be born again.
You can't see God unless the Holy Spirit (Luke 10:21) reveals Him, but all you have to do is ask (Luke 11:13). Tell Him you want to come to Him on His terms (Romans 3:23, Acts 3:19, John 6:28-29) and accept the gift of pardon He has freely offered you through His Son, Jesus Christ (Romans 10:9-12). Once you do this, the Father will open your eyes (Acts 26:18) through His Spirit and you will be born again!
For a more exhaustive explanation, download Chuck Smith's How Can A Man Be Born Again?