Alan Kerr (The Temple of Jesus’ Body: The Temple Theme in the Gospel of John (Library of New Testament Studies), 71) offers this comment on Jesus’ statement that Nicodemus had to be born of the Spirit before entering the kingdom: “It is almost universally accepted that Spirit here refers to the Spirit of God. But at this stage in the Gospel there was no Spirit (7:39), because Jesus was not yet glorified. It is not until Jesus is risen and appears to the disciples and breathes on them and says, ‘Receive the Holy Spirit’ that the Spirit is given (20:22). So from the point of view of Johannine timing what Jesus says to Nicodemus should only be realized in a post-resurrection setting. Properly speaking he can only be reborn from above when Jesus is glorified.”
This obviously affects the use of John 3:5 as a proof text for the doctrine of regeneration.