Heaven is forever (see Revelation 22:5). In order for Heaven to be Heaven, there can never be the nagging possibility of a revolt which would cause us to lose the glory of it again. There will be no doubt, no confusion and no fear in heaven. God will remove from the scene the cause of all rebellion–Satan and sin.
Consequently, God cannot allow people who rebel against Him to enter Heaven. Everyone must be born again before he or she can enter Heaven. Anyone who has not been transformed in his spirit would be a source of spiritual contagion, contamination or rebellion in Heaven.
A little tendency to sin today could grow into a hideously dangerous nature in, say, a million years. Therefore, the seed of rebellion will never be allowed to enter Heaven.