A person will not be allowed into the Kingdom of God apart from the new birth.  As the population increases during the Kingdom Age, there will be more and more individuals who are not getting saved, especially near the end of the 1,000 year reign of Jesus Christ.  During this time, the knowledge of the Lord will be known by everyone.  But, after Satan is loosed from the Bottomless Pit, he will deceive the nations of the world, bringing them against the great King, Jesus Christ, God in the flesh.  Even though these rebels knew better, they still came against Him, knowing that fire would come down out of Heaven to devour them.  This has got to be the most surprising thing in the entire Bible.