Since this question comes up a lot, and it can be pretty dense to try and explain, I tend to repost an explanation I've given numerous people. I hope you don't mind.
And if anything I said below doesn't make sense, feel free to ask questions for clarification! I don't bite.
Before we continue, the following two disclaimers need to be made:
1: When we Christians speak of the Trinity, we do NOT define it as God having schizophrenia or multiple-personality disorder; the Trinity is not God switching between three different "modes" or "masks."
2: When we Christians speak of the Trinity, we do NOT mean to say that Father, Son and Holy Spirit are three wholly separate entities. We do not have three Gods, but One.
Now that that's out of the way, here is the actual definition of what the Trinity is, courtesy of
OrthodoxWiki:
Orthodox Christians worship the Father, Son, and Holy Spiritthe
Holy Trinity, the one God. Following the Holy Scriptures and the Church Fathers, the Church believes that the Trinity is three divine persons (hypostases) who share one essence (ousia). It is paradoxical to believe thus, but that is how God has revealed himself. All three persons are consubstantial with each other, that is, they are of one essence (homoousios) and coeternal. There never was a time when any of the persons of the Trinity did not exist. God is beyond and before time and yet acts within time, moving and speaking within history.
God is not an impersonal essence or mere "higher power," but rather each of the divine persons relates to mankind personally. Neither is God a simple name for three gods (i.e., polytheism), but rather the Orthodox faith is monotheist and yet
Trinitarian. The God of the Orthodox Christian Church is the God of Abraham, Isaac and Jacob, the
I AM who revealed himself to Moses in the burning bush.The source and unity of the Holy Trinity is the Father, from whom the Son is begotten and also from whom the Spirit proceeds. Thus, the Father is both the ground of unity of the Trinity and also of distinction. To try to comprehend unbegottenness (Father), begottenness (Son), or procession (Holy Spirit) leads to insanity, says the holy Gregory the Theologian, and so the Church approaches God in divine mystery, approaching God apophatically, being content to encounter God personally and yet realize the inadequacy of the human mind to comprehend Him.
Now, to define those Greek terms that showed up in that quote:
-Hypostasis: A person, or a subsistence/manifestation/arising of an essence
-Ousia: An essence; i.e. that which makes an entity that particular entity; for example, the essence of Shiranui117 is different from the essence of Joe Schmoe.
-Homoousios: Of one essence.
In this case, since Father, Son and Holy Spirit are all of the same Divine Essence, they all are one "being," each Person being fully God. It is not a case of 1/3+1/3+1/3=1, or of 1+1+1=3, but 1+1+1=1. Father, Son and Holy Spirit are not just "parts" of God, but are each fully God in their own right. Three distinct Persons, yet one God. Distinct, yet not separate. United, yet not confused or mixed.
This is from Metropolitan Kallistos Wares book The Orthodox Way. (pp. 29-31)
The central and decisive affirmation in the Creed is that Jesus Christ is true God from true God, one in essence or consubstantial (homoousios) with God the Father. In other words, Jesus Christ is equal to God the Father; he is God in the same sense that the Father is God, and yet they are not two Gods but one. Developing this teaching, the Greek Fathers of the later fourth century said the same about the Holy Spirit; he is likewise truly God, one in essence with the Father and the Son. But although Father, Son and Spirit are one single God, yet each of them is from all eternity a person, a distinct centre of conscious selfhood. God the Trinity is thus to be described as three persons in one essence. There is eternally in God true unity, combined with genuinely personal differentiation: the term essence, substance or being (ousia) indicates the unity, and the term person (hypostasis, prosopon) indicates the differentiation. Let us try to understand what is signified by this somewhat baffling language, for the dogma of the Holy Trinity is vital to our own salvation.
Father, Son and Spirit are one in essence, not merely in the sense that all three are examples of the same group or general class, but in the sense that they form a single, unique, specific reality. There is in this respect an important difference between the sense in which the three divine persons are one, and the sense in three human persons may be termed one. Three human persons, Peter, James and John, belong to the same general class man. Yet, however closely they co-operate together, each retains his own will and his own energy, acting by virtue of his own separate power of initiative. In short, they are three men and not one man. But in the case of the three persons of the Trinity, such is not the case. There is distinction, but never separation. Father, Son and Spiritso the saints affirm, following the testimony of Scripturehave only one will and not three, only one energy and not three. None of the three ever acts separately, apart from the other two. They are not three Gods, but one God.
Yet, although the three persons never act apart from each other, there is in God genuine diversity as well as specific unity. In our experience of God at work within our life, while we find that the three are always acting together, yet we know that each is acting within us in a different manner. We experience God as three-in-one, and we believe that this threefold differentiation in Gods outward action reflects a threefold differentiation in his inner life. The distinction between the three persons is to be regarded as an eternal distinction existing within the nature of God himself; it does not apply merely to his exterior activity in the world. Father, Son and Spirit are not just modes or moods of the Divinity, not just masks which God assumes for a time in his dealings with creation and then lays aside. They are on the contrary three coequal and coeternal persons. A human father is older than his child, but when speaking of God as Father and Son we are not to interpret the terms in this literal sense. We affirm of the Son There was never a time when he was not. And the same is said of the Spirit.
. . .Each possesses, not one third of the Godhead, but the entire Godhead in its totality; yet each lives and is this one Godhead in his own distinctive and personal way.
Elsewhere in the book, the Metropolitan speaks of the relationship between various members of the Trinity. I will summarize it as follows:
-The Father is the source (Greek: arche) of the Trinity, and so is in a certain sense "greater" than the Son and Spirit. Not by virtue of being more God than the other two, but by virtue of being the source of the Trinity. It is from Him that the Son is begotten and it is from Him that the Holy Spirit proceeds.
-The Son is eternally begotten of the Father, and is also called the "Logos" or "Word" of the Father. The traditional interpretation of this is that the Son is the Word by which God spoke the world into being. The Son is not a created being, for as it says in John 1:2-3, "He was in the beginning with God. All things were made through Him, and without Him nothing was made that was made."
-The Holy Spirit proceeds from the Father, or from the Father THROUGH the Son (The Latin filioque, "proceeds from the Father AND the Son", is not original and toes the line of heresy). Yet the Holy Spirit is not subordinate to the Father or the Son. As for how "being begotten" is different from "proceeding from," we cannot say.
When speaking of God's essence, we Trinitarians do not refer to "essence" as a general class or nature; for example, a human nature. Rather, we speak of God's Essence as signifying "the whole God as he is in himself," as Ware said. This shows an essence that is unique to God alone; for instance, I am different from you in how I am within myself, and you are different from me in how you are in yourself.
The Antiochian Orthodox Archdiocese has
a page about this, too.
The Orthodox Church in America has a primer about this online for free.
Here's that link.
Indeed it IS. It's a lie promulgated by the Holy Roman Church to gain pagan converts inspired by Satan the great deceiver.
It's roots are pagan. The triune godhead is false Christian doctrine and you would be well advised to do some reseach on the trinity for yourself as I did.
Obviously you didn't research very hard or look at primary source documents from the earliest Christians. We have stuff from personal students of St. John the Apostle, one of whom became bishop of the Church that St. Peter founded in Antioch, saying explicitly that Jesus is God.