Jesus answered him, "Most assuredly, I tell you, unless one is born anew,{The word translated "anew" here and in John 3:7 (anothen) also means "again" and "from above".} he can't see the Kingdom of God."
The kingdom of God - Either in this world or in that which is to come - that is, heaven. See the notes at Matthew 3:2. The meaning is, that the kingdom which Jesus was about to set up was so pure and holy that it was indispensable that every man should experience this change, or he could not partake of its blessings. This is solemnly declared by the Son of God by an affirmation equivalent to an oath, and there can be no possibility, therefore, of entering heaven without experiencing the change which the Saviour contemplated by the “new birth.” And it becomes every man, as in the presence of a holy God before whom he must soon appear, to ask himself whether he has experienced this change, and if he has not, to give no rest to his eyes until he has sought the mercy of God, and implored the aid of his Spirit that his heart may be renewed.