The reality of the Spirit

John 3:7-8 Do not marvel that I said to you, 'You must be born again.' The wind blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit."

Nicodemus objects to the idea of being born again because he doesn't understand how it can be. Jesus shows him that that is not a valid reason for objecting to it, because he constantly believes things that are just as difficult to understand. It might appear to make no sense, but the reality of the doctrine was to be judged by its effects. As in the case of the wind, we can feel its effects, we can hear its sound, we can see important changes that are produced by it -- trees and clouds are moved. Yet the wind is not seen, nor do we know from where it originates, nor what laws govern it. So it is with the operation of the Spirit. We see the changes produced. That is evidence enough, we don't have to understand the "how" in order to accept its reality.