Who is the Holy Spirit? On this day, Trinity Sunday, we Christians celebrate and recognize that God is One, but is also One in Three: Father, Son, and Holy Spirit. This is a central truth to our Faith, but certainly a difficult and mysterious truth. Today we will begin a series focusing on the Third Person of the Trinity, the Holy Spirit, and on understanding the Holy Spirit as the Lord the Giver of Life, the very breath of God who enacts and fulfills the promises, presence, and power of God.