Jesus answered and said unto him, Truly, truly, I say unto you, 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.

Heaven is the Spirit Realm.   

A king presides over a kingdom.   
Christ Jesus is King of heaven.   

Being Born Again is being born into the Spirit Realm by the Word and Holy Spirit.   

Once a person believes and receives the Gospel of Jesus Christ, they become a New Spirit being who is a citizen of the Kingdom Of Heaven.   

That New Spirit being is in the Kingdom Of Heaven but its soul is still floundering in ignorance and error.   

For the soul to be saved, it must continue to pursue righteousness Mat 6:33 .   

That pursuit involves prayer and exposure to the Word Of The Lord 2Ti 2:15 Eph 6:18 Jud 1:20 .   

As a believer pursues righteousness their eyes are opened to the provisions of the kingdom into which they have been born Eph 1:18 .