The first thing to note about the Holy Spirit is that it is absence- the absence of powers that enslave a person, whether these are compulsions internal to a person, deriving from trauma visited upon them, or external forces of society, culture or religion. The first apostles for example were unbound from the guilt of having deserted Jesus, and of their own ordinary sins. They were liberated from race, language, nationality, and from the compulsions of wealth. Above all, the spirit of individualised life was absent and in its place was communion, koinonia, shared life.
The Holy Spirit, which is God’s spirit, is not some kind of supernatural gloop which enters into a person, nor is it a secret power which determines positive emotions and beliefs. It is the absence of false determinants and permits the person to live and move and have their being in God. It is the ministry of Jesus which enables a person to die to the powers that control the world and worldly people, and to rise into the liberating spirit of God, which does not enslave, but invites people to live as children of God. They are persuaded to live as God intended.
It is important to reiterate that the Holy Spirit is no-thing. It is not something of the same sort as any worldly thing. It is God, an undefinable reality. The Johannine tradition names it as love and koinonia (shared life). Paul names it as the spirit of adoption by which we cry out to God, Abba, dear father. Although it enters individual persons, it makes them no longer isolated individuals but members of the ecclesia, the assembly of brothers and sisters, citizens of God.
As is evident from my own attempts here, it cannot be fully described, because it is not a reality of this world. Like judgement (see above) it is an eschatological reality, which will only be fully known in the completion of God’s creation. It comes to human beings from the future and is creative of the future. But it is not unknowable: it is known by its fruits, which Paul describes as love joy peace, patience, kindness, goodness, faithfulness, gentleness and self-control.