[Oneanddone]

Whose fault was it Nicodemus had not already been born again when the conversation took place in Jn 3?

If Nicodemus had no role in his own new birth, how could he be criticized for not being born again?

[MHGinTN]

To answer your queries you must first know what it means to be born from above / born again. Without that information the answers to your questions will not be understood by you.

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.