The aliens on this planet treat our astronauts with grace and hospitality when they arrive. And after a very short time it becomes clear to the astronauts that this is the place that we humans need to move to - it is literally perfect. However, the aliens have one very strict law which states that the only way to become a citizen of their planet and live there is if you are born there. What a bummer.
So, as it is with us humans, we beg and plead and try our best to get the aliens to change the law - but they can't - it's not possible. We even try looking just like the aliens (they aren't little and grey with big eyes), we dress like them, learn to talk like them, walk like them, but it just can't be done. We cannot live on their planet.
So it is with the kingdom of heaven.
Put on all you like of Christianity, appear in every way Christian. However, if you are not born again of the spirit - then heaven is closed to you.
A person may do wonderful and caring works, spend their life in altruistic pursuits. If they are not born again of the spirit - then heaven is closed to them.
A reprobate in the clutches of the sins of alcoholism and addiction may strive valiantly and reform themselves. They may turn away from their destructive vices and lead a commendable life as a fine member of society. But if they are not born again of the spirit - then heaven is closed to them.
A christian is not a Christian without a Christian heart. And for a heart to be Christian it must be born again of the spirit in the heavenly country. To see and dwell in the kingdom of God we must be born anew there.
This may seem like a frightening and confrontational message - but it is the message the Lord gives us:
"Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God." - Jesus (John 3:5)
There is no wiggle room.