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 term "kingdom of God" as Jesus used it consistently refers to the earthly messianic kingdom that will be the earthly phase of God’s eternal heavenly kingdom. To enter the kingdom of God means to obtain eternal life (cf. Mark 9:43; Mark 9:45; Mark 9:47). John used "kingdom" language rarely (John 3:3; John 3:5; John 18:36). This is the only passage in John that mentions the kingdom of God, though Jesus spoke of "my kingdom" in John 18:36. He used "life" language instead (cf. John 1:12-13). This is understandable since he evidently wrote late in the first century when it was clear that God had postponed the kingdom. His readers needed to prepare for the future immediately by obtaining eternal life.