To answer the question, what is the spirit mentioned in John 3:6, we have to understand that Jesus is replying to the question hidden in the non-question statement of Nicodemus: “Rabbi, we know that You have come from God as a teacher; for no one can do these signs that You do unless God is with him.”
Is there really a question in that statemen? Sure, Nicodemus is being coy, and what he really wants to know is what Jesus's intention is.
Jesus tells him he is not surprised that Nicodemus cannot understand his motives, because only those who have been baptised by the Holy Spirit and fire, like his disciples, can understand and access the Kingdom of God, which He has ushered in.
These are the steps that are gone through, to access the Kingdom, the motifs which are seen in both the Old, as well as the New, Testament.
Believers confess that selfish living is harmful, come out of Egypt, are willing to consider the alternative provided by God. This alternative is to live selflessly, and safely, because God is involved, His finger is in the situation:
Exodus 8:19 Then the magicians said to Pharaoh, “This is the finger of God.” But Pharaoh’s heart was hardened, and he did not listen to them, as the LORD had said.
Luke 11:20 But if I drive out demons by the finger of God, then the kingdom of God has come upon you.
When believers see the great work of God, they SEE the Kingdom of God, God working with humans to subdue creation, cause it to turn to God, thus being blessings to the world. This is how this submission works :
Joshua 2:11"When we heard it, our hearts melted and no courage remained in any man any longer because of you; for the LORD your God, He is God in heaven above and on earth beneath.
John 3:1Now there was a man of the Pharisees named Nicodemus, a leader of the Jews. 2He came to Jesus at night and said, “Rabbi, we know that You are a teacher who has come from God. For no one could perform the signs You are doing if God were not with him.”
Those who receive this insight bury it deep in their hearts, so that it takes root and grows to have faith, so that when they hear God's voice, they will not be like Israel, who because of lack of faith, did not enter the Kingdom.
Let us regroup:
Those who have been baptised by the Holy Spirit and with fire are shown great works of God working with believers, and understanding of how this helps them become blessings to the world. They have a choice to neglect these revelations and understandings and not believe, or nurture them so that they will have greater faith, have a different spirit, be born again, so that when they hear God's voice, they can obey and be blessings to the world.
Numbers 14:24But because My servant Caleb has had a different spirit and has followed Me wholeheartedly, I will bring him into the land he has entered, and his descendants will inherit it.
Now let's form the conclususion:
This is how the giving of the Holy Spirit leads to the formation of a spiritual believer, just as living in the world, flesh, leads to the formation of a worldly believer.
Nicodemus, in turn, needs to first believe Jesus is the Messiah, as taught by John the Baptist, so that he can be given the baptism in the name of Jesus, leading to the giving of the Holy Spirit, in turn witnessing God working with humans, to subdue creation. Only then can his worldly self be reborn spiritually, depending on how he nurtures the revelation, the word, the communication from God.
Luke 8:8“Other seed fell into the good soil, and grew up, and produced a crop a hundred times as great.” As He said these things, He would call out, “He who has ears to hear, let him hear.”9His disciples began questioning Him as to what this parable meant. 10And He said, “To you it has been granted to know the mysteries of the kingdom of God, but to the rest it is in parables, so that SEEING THEY MAY NOT SEE, AND HEARING THEY MAY NOT UNDERSTAND.
11“Now the parable is this: the seed is the word of God. 12“Those beside the road are those who have heard; then the devil comes and takes away the word from their heart, so that they will not believe and be saved. 13“Those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. 14“The seed which fell among the thorns, these are the ones who have heard, and as they go on their way they are choked with worries and riches and pleasures of this life, and bring no fruit to maturity. 15“But the seed in the good soil, these are the ones who have heard the word in an honest and good heart, and hold it fast, and bear fruit with perseverance.
Matthew 23:24“And the one also who had received the one talent came up and said, ‘Master, I knew you to be a hard man, reaping where you did not sow and gathering where you scattered no seed. 25‘And I was afraid, and went away and hid your talent in the ground. See, you have what is yours.’
Matthew 25:3“For when the foolish took their lamps, they took no oil with them, 4but the prudent took oil in flasks along with their lamps.
All Scripture from the NASB.