Who is the Holy Spirit? What is the Holy Trinity?
In short: Christians do not believe in three gods, but in One alone, who is Father, Son, and Holy Spirit. One God, three Persons, who love each other perfectly and are one. The Holy Spirit is not a force or a vague energy, He is God Himself, the One who works in the Church and in your heart and brings you closer to Christ.
The Orthodox nuance
First, what the Trinity is not. They are not three gods, that would be paganism. And it is not a single God who puts on three masks in turn, sometimes Father, sometimes Son, sometimes Spirit, that was rejected as an error. We confess one single God, one single essence, but three true and distinct Persons, from all eternity.
How does this hold together? We do not fully grasp it with our minds, and it is honest to say so, it is a Mystery. But its core is love. God is love, says Saint John (1 John 4, 8), and love cannot be alone, it needs someone to love. From all eternity the Father loves the Son, in the Holy Spirit. God did not become love when He made the world, He already was, in Himself, a communion of love.
And the Holy Spirit is often the least understood. He is not a mist and He is not just the power of God. He is a Person, the "Lord, the Giver of Life," as we say in the Creed. He spoke through the prophets, He descended at Pentecost, He sanctifies the water of baptism and transforms the gifts at the Liturgy. When you pray and feel a warmth, a clarity, a power to forgive beyond your nature, that is Him working.
Sources
- Matthew 28, 19 (baptize them in the name of the Father, and of the Son, and of the Holy Spirit)
- 1 John 4, 8 (God is love)
- John 15, 26 (the Spirit of truth, who proceeds from the Father)