In John 3, Jesus told Nicodemus that only people who are born again will see/enter the kingdom of God (3:3, 5).
The four Gospels (especially John’s), and the New Testament in general, presents the reader with an “already-but-not-yet” tension regarding kingdom realities like new birth, resurrection, eternal life, justification, etc. These blessings are not relegated to the day of the kingdom’s consummation but are partially realized and experienced with the kingdom’s inauguration in Jesus.
Jesus wasn’t telling Nicodemus that people who are born again will one day enter the kingdom–they will enter the kingdom now with the gift of eternal life, and they will enter the kingdom now with the gift of regeneration.
This means that when people are born again they are citizens of the kingdom of God. Born again now, people of the kingdom now.