God is One. The Orthodox Christian doctrine of the Trinity states that God is one in divine essence and eternally exists as three distinct persons: the Father, the Son, and the Holy Spirit. Three hypostases (persons); one divinity.
God the Father is the first person of the Trinity. The Father is the ‘fountain’ of the Godhead, the source of divinity, the cause or principle of origin for the other two persons. He is the bond of unity between the three: there is one God because there is one Father. “The union is the Father from whom and to whom the order of the persons runs its course.” (St. Gregory Nazianzus)
The other two persons are each defined in terms of their relationship to the Father: the Son is ‘begotten’ by the Father, the Spirit ‘proceeds’ from the Father.
The Son of God, Jesus Christ, is the second person of the Holy Trinity. He is the ‘Word’ or ‘Logos’. It is in and through the son that the Father is revealed to us. God became man in the person of Jesus, who said, “I am the Way, the Truth, and the Life: no one comes to the Father, except through me.” (John 14:6)
He was born on earth as man, from the Virgin Mary in the city of Bethlehem. But as Word of God he is also exists before the Incarnation. It is in Christ Jesus that we are saved, that is, through his sacrificial death on the Cross and our belief in him as Lord.
The third person is the Holy Spirit, the ‘wind’ or ‘breath’ of God. We understand the Holy Spirit best from the prayer: “O Heavenly King, the Comforter, the Spirit of Truth, who art everywhere present and fillest all things, Treasury of blessings and Giver of life: come and abide in us, and cleanse us from every stain, and save our souls, O gracious Lord.”
Appreciating the inadequacy of neat classifications, we may say that the Spirit is God within us, the Son is God with us, and the Father God above or beyond us. Just as the Son shows us the Father, so it is the Spirit who shows us the Son, making him present to us. Yet, the relationship is mutual. The Spirit makes the Son present to us, but it is the Son who sends us the Spirit. The Spirit is sent into the world, within time, by the Son; but as regards his origin within the eternal life of the Trinity, the Spirit proceeds from the Father alone.