Here's a big hint: the kingdom Age would not have been hallmarked by The Holy Spirit in the spirits of Gentiles. The Church Age is the Age of Grace, the Age of The Righteousness of Christ imputed to ALL believers both Jew and Gentile. No works are required for the God to impute the righteousness to an individual spirit, but during the Kingdom Age there will be Temple functions and works to fulfill.
The reason Nicodemus had not been born again was his own fault for man has a role to play in his own new birth.
At that time in Jn 3, John’s water baptism was in effect (Jn 3:23), John’s baptism would remit sins (Mk 1:4) yet Pharisees as Nicodemus rejected John’s baptism (Luke 7:30).
John’s baptism has been replace by Christ’s water baptism of the great commission (Mt 28:19,20 > Acts 2:38). Those that submit to Christ’s baptism (born of water) have all sins remitted then can walk in newness of life (Rom 6:4).
Those who submit to Christ’s baptism are baptized into Christ therefore are clothed in Christ (Gal 3:27). Having put on Christ in baptism, one is covered by Christ’s perfect righteousness and therefore seen by God as perfectly righteousness. Christ’s righteousness is not transferred to the sinner while the sinner sits idle. But the sinner obeys the will of God in submitting to baptism and then is born again having put on Christ.