When talking to the Jewish religous leader, Nicodemus, Jesus told him that unless he was born again he could not see the kingdom of God.  Jesus made being born again a requirement for gaining an entrance into heaven.  It was not an arbitrary requirement.  Jesus didn't just decide to tack that on to the list of objectives for getting into heaven.  The requirement for the new birth was made necessary due to the justice of God.  God requires a just punishment for sins committed, and Jesus satisfied that requirement for believers by his death upon the cross.  God also demands a righteous life and Jesus provided his righteousness as a substitution for our defective righteousness.  All is good so far, but it is still not enough for an entrance into heaven.  Something has been done with the sin (Jesus paid the penalty), and something has been done for God's demand of perfect righteousness (by providing his righteousness) - but nothing has been done with the sinner.  Nothing has been done with the heart that produced the sin.  A liar can be forgiven his lies, but a heart that produced lies on earth (unless changed) will produce lies in heaven.  God will not have it that way and therefore demands a radical change of heart.  Fortunately for us the God that demands a change also provides that change - and Jesus called it being born again.   "Ye must be born again."  John 3:3