First of all, it is the Church’s teaching and its deepest experience that there is only one God because there is only one Father.

In the Bible the term “God” with very few exceptions is used primarily as a name for the Father. Thus, the Son is the “Son of God,” and the Spirit is the “Spirit of God.” The Son is born from the Father, and the Spirit proceeds from the Father — both in the same timeless and eternal action of the Father’s own being.

In this view, the Son and the Spirit are both one with God and in no way separated from Him. Thus, the Divine Unity consists of the Father, with His Son and His Spirit distinct from Himself and yet perfectly united together in Him.

Now here we have to see a very important point for Trinitarian theology. And that is in the Bible, in the Scriptures, and then, therefore, in the Creeds — and particularly in the Niceno-Constantinopolitan Creed, which became the creedal statement for ancient Christianity and remains the baptismal, liturgical creed for the Eastern Orthodox Churches and most Christian Churches for this very day, as it was formulated and put together and received from the first two Ecumenical Councils (Nicaea in 325 and Constantinople in 381) — that [is] in this Creed and as it is proclaimed in liturgical prayers — and certainly in the Liturgical Prayer, the Anaphora (which is a word that means «raising up» or «offering up», which is a technical term for the Eucharistic prayer, the Eucharistic canon, where the bread and wine, the prosphora, are first elevated and offered to God as we lift up our hearts and have our hearts on high when we remember the saving activity of Christ at the Holy Eucharist service) — in the Bible, in the creeds, and in the Liturgy, it's very important, really critically important, to note and to affirm and to remember, that the one God, in Whom we believe, strictly speaking, is not the Holy Trinity. The one God is God the Father. That in the Bible, the one God is the Father of Jesus Christ. He is God Who sends His only-begotten Son into the world. And Jesus Christ is the Son of God. And then, of course, in a parallel manner, the Spirit, the Holy Spirit is the Spirit of God. That the Holy Spirit being the Spirit of God, is therefore also the Spirit of Christ, the Messiah, because the Christ is the Son of God, upon Whom God the Father sends and affirms His Holy Spirit. I think this is very important, because there are wrong understandings of the Holy Trinity.

Jesus is born from the Virgin Mary because he is the divine Son of God, the Saviour of the world. It is the formal teaching of the Orthodox Church that Jesus is not a “mere man” like all other men. He is indeed a real man, a whole and perfectly complete man with a human mind, soul and body. But he is the man which the Son and Word of God has become. Thus, the Church formally confesses that Mary should properly be called Theotokos, which means literally “the one who gives birth to God.” For the one born of Mary is, as the Orthodox Church sings at Christmas: “… he who from all eternity is God.”

Jesus of Nazareth is God, or, more accurately, the divine Son of God in human flesh. He is a true man in every way. He was born. He grew up in obedience to his parents. He increased in wisdom and stature (Lk 2.51–52). He had a family life with “brethren” (Mk 3.31–34), who according to Orthodox doctrine were not children born of Mary who is confessed as “ever-virgin,” but were either cousins or children of Joseph.

On the other hand, there is another terrible error, and the other terrible error, usually called Modalism in technical theological terminology, is where people say: there is «one God Who is the Holy Trinity», there is «He Who Is the Trinity». And we Orthodox Christians, following scripture, and the creedal statements, and the liturgical prayers, can never say: there is «one God Who is the Holy Trinity». There is «one God Who is the Father». And this one God Who is the Father has with Him eternally, Whom He begets timelessly before all ages, His only-begotten Son — Who is also His Logos, His Word, and also His Chokhmah, His Sophia, His Wisdom, also His Eikona, His Ikon, His Image. But this Wisdom and Word and Image and Ikon of God is divine with the very same divinity as God, the One True and Living God, because «He is Who He is», and His is another Who from the Father. There are three Whos. There is He Who is the Father, He Who is the Son, and He Who is the Holy Spirit. Those three Whos are called the three Persons or three Hypostases. Probably the term «hypostases» is a better term, because it means three instances of divine life in a perfect and total unity. But it is important to remember that the one God is the Father of Jesus. Jesus is the Son of God. As the Nicene Creed said, “He is God from God, true God from true God”. Here the Christians would say and insist that the one God and Father, from all eternity, has with Him His Son.