When Adam sinned the spirit of man was forever corrupted.   He died spiritually and all his progeny were born Physically alive but Spiritually dead.   

But the Lord commendeth his love toward us, in that, while we were yet sinners, Christ died for us. Rom 5:8 No one can see our Heavenly Father, the Holy Spirit.     Since man is carnal and He is Spirit, no one could really know Him Joh 17:25.      Prophets had spoken of Him and some aspects of His nature were revealed.     But the perfect Truth of His love and His desire for us was not revealed and could not be understood.      So, He poured Himself into the body of Christ Jesus physically, mentally and spiritually Mat 1:18.      His Word became flesh and dwelt among us Joh 1:14.      While we were separated from Him in the pathetic state of spiritual death, the perfect Son Of Man, the Son Of The Holy Spirit shed His blood to redeem us.

Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come. Rom 5:14 The Sin Nature destroyed the spiritual life that the Lord had put in man.

Jesus told Nicodemus ...   

Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of heaven. Joh 3:3 Human beings are born with the Sin Nature - a corrupt spirit.     That corrupt spirit is dead to the Lord Gen 2:17.      By being Born Again of the Holy Spirit we can begin to see (understand and appreciate) the Kingdom Of God.

Marvel not that I said unto thee, Ye must be born again. Joh 3:7 This is the first time Nicodemus has heard about being Born Again.

Nicodemus was a Pharisee who did as good a job as anyone on earth to manage their Evil Hearts.   
Jesus was telling him that adherence to a Moral code just does not cut it.   

He was telling Nicodemus that the Sin Nature in man had to be destroyed and that a New Nature replace it.   

Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. 2Co 5:17 When we come to Christ Jesus in faith that He is Mighty God, Everlasting Father, Saviour and Lord we are born of His Holy Spirit.      At that point we are a New Creature.     We are super-human.     We are Sons Of The Holy Spirit.      But the new creation is hidden with Christ in the Holy Spirit.      The only way that it manifests is through a transformed mind.      We do not get the transformed mind at the point of faith.     We get it by absorbing the Word Of The Lord and meditating on it in the Holy Spirit.     We must then walk out that which is in our mind and heart.      Old things have passed away and all things have become new as far as the Lord is concerned, but we must forget the past and see ourselves as the Lord sees us for the effect of the old things to pass away from our minds and hearts.      We must choose to see all things as new in order to experience new things.

There is a major difference between sinning and being a sinner.   We all fall short of the Glory of God.   However when we are Born of the Word and Holy Spirit, our desire is to please Him.   

What remains in a Believer after being Born Again of the Word and Holy Spirit is a untransformed Mind that must be renewed.   

As we allow the Word to wash our Minds we are able to manifest the good, acceptable and perfect will of God.   

And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of the Lord. Rom 12:2 What is wrong with the world that we should not be conformed to it?         → it is lost and confused    → it is filled with sickness and disease    → it is filled with violence    → it is filled with greed    → it is filled with hate    → it is filled with death    → it is hopeless    → it is apart from the Lord Conformity is good when it is to the Word and the Holy Spirit.     Otherwise it is evil.      In order for the New Spirit being to manifest in our flesh, we must be transformed in mind, emotions and will (our soul).      When we are transformed we receive the wisdom and power of the Lord to do the work of Christ Jesus in our mortal bodies.      What is the good and acceptable and perfect will of our Lord?         → It is that none perish 2Pe 3:9,    → it is that all be healed 1Pe 2:24,    → it is that all prosper 3Jn 1:2.      How is the Lord's will proved?      By the manifestation of evidence.      What evidence?      salvation, healing and prosperity.      Hence, the only thing holding us back from experiencing the best that the Lord has for us is our minds.      Let the Mind Of Christ be in you.     - Phi 2:5