I suspect that the religious leaders of Jesus’ day think that they have him worked out – and so, they have God and his kingdom worked out. They have looked at what Jesus is doing. They have assessed him. And they have boxed him – and God.
As Nicodemus lays all this out before Jesus, Jesus turns it all on its head. First, he points out that it is only the work of God that can bring anyone into God’s kingdom. Second, this is how God has always promised to work. And third, this will create a distinction amongst humans that will last forever, starting now.
In many ways, this is the passage that lies behind the little taster we received in the hallway into John’s house, in John 1:12-13: all of this is by God’s work, and not by the will of humans, the heritage of humans, or the deserving nature of humans.