The Trinity is a central doctrine of most branches of Christianity; it says that God is one God, existing in three distinct persons, usually referred to as Father, Son and Holy Spirit. Historically, this has been described by the Nicene (325 A.D.), Apostles’ (200 A.D.), and Athanasian Creeds (mid 300’s A.D.) although it is not explicitly described in the Bible. These creeds were created and endorsed by the orthodox, catholic Church of the third and fourth centuries, and later retained in some form by most Protestants.
The Nicene Creed, which is a classic formulation of this doctrine, used “homoousia” (Gk of same substance) to define the relationship among the members of the Godhead. The spelling of this word differs by a single Greek letter, “one iota”, from the word used by non-trinitarians at the time, “homoiousia”, (of similar substance): a fact which has since become proverbial, representing the deep divisions occasioned by seemingly small imprecisions, especially in theology.
Scripture and tradition
The word, Trinity, literally means, “a unity of three”. This word does not appear in the Bible, and indeed, it apparently did not exist until Tertullian coined the term in the early third century. Nevertheless, although trinitarian Christians grant that the modern words and formulas are later developments, they still believe that this doctrine is found systematically throughout the Bible, and in the creeds and doctrines, and in other traditions of the Christian Church. It is considered a biblical doctrine “only on the principle that the sense of Scripture is Scripture”. 
Belief in God as a Trinity is considered essential by Eastern Orthodoxy, Roman Catholicism, and orthodox Protestantism. However, Christian faith does not ask for comprehension: it must be understood that God is a Trinity, for the sake of knowing who God is, and for understanding the salvation he has accomplished. Beyond such practical issues, speculation regarding a theory of the divine being is not necessarily encouraged. The believer does not need to know how it is that God is a Trinity; and in fact, that issue is more often taught in terms of what the Trinity is not, distinguishing the doctrine from the many alternatives.
Baptism as the practical starting point
In practice, what a Christian begins to learn about the Trinity starts with Christian Baptism. This is also the starting point to apprehend why the doctrine matters to so many Christians, even though what the doctrine teaches about the being of God is beyond complete comprehension. The Apostles’ Creed, for example, has been commonly used as a brief summation of Christian faith, to be professed by converts to Christianity when they receive baptism, and at other times in the liturgy of the church.
Christians are baptized “In the name of the Father, and of the Son, and of the Holy Spirit (Matt 28:19). Christian life, and the Christian understanding of salvation, begins with a declaration of the Trinity. Basil the Great ( 330-379) explains:
“We are bound to be baptized in the terms we have received, and to profess faith in the terms in which we have been baptized”
At the baptism of Jesus Christ, the Trinity appears: “And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, This is my beloved Son, with whom I am well pleased.” (Matthew 3:16-17, RSV). To trinitarians the three persons of the Trinity were made manifest at once, in connection with baptism. “This is the Faith of our baptism”, the council of Constantinople declared ( 382), “that teaches us to believe in the Name of the Father, of the Son and of the Holy Spirit. According to this Faith there is one Godhead, Power, and Being of the Father, of the Son, and of the Holy Spirit”.
Christian Life and the Trinity
This singleness of God’s being, and mysterious multiplicity of persons, accounts for the nature of Christian salvation, and discloses the gift of eternal life. “Through the Son we have access to the Father in one Spirit” (Eph 2:18). This communion with the Father is the goal of the Christian faith, and is eternal life. It is achieved through God’s union with human nature, in Jesus Christ who although fully God, humanly died for sinners to purchase their redemption; and this forgiveness and friendship with God is made accessible through the gift to the church of the Holy Spirit, who raised Christ from the dead, and who, being God, knows God intimately and leads and empowers the Christian to fulfill the will of God. Thus, this doctrine touches on every aspect of the trinitarian Christian’s faith and life; and that accounts for why it has been so earnestly contended for, throughout Christian history, despite the difficulty inherent in explaining the doctrine.
God is a single being. The Hebrew Bible lifts this one article of faith above others, and surrounds it with stern warnings against departure from this central issue of faith, and of faithfulness to the covenant God had made with them. “Hear O Israel! The Lord our God is One God” (Deut 6:4), “You shall have no other gods” (Deut 5:7) and, “This is what the LORD says- Israel’s King and Redeemer, the LORD Almighty: I am the first and I am the last; apart from me there is no God.” (Isaiah 44:6). Any formulation of an article of faith which does not insist that God is solitary, that divides worship between God and any other, or that imagines God coming into existence rather than being God eternally, is not capable of directing people toward the knowledge of God, according to the trinitarian understanding of the Old Testament. The same insistence is found in the New Testament: “there is no God, but one” (1 Corinthians 8:4). The “other gods” warned against are therefore not gods at all, but unequal substitutes for God.
So, in the trinitarian view, the common conception is a profoundly mistaken one, which thinks of the Father and Christ as two separate beings. The central, and crucial affirmation of Christian faith is that there is one savior, God, and one salvation, in Jesus Christ, to which there is access only because of the Holy Spirit. The God of the Old is the God of the New, and alone is God.
God exists in three persons
This one God however exists in three persons, or in the Greek hypostases. God has but a single divine nature, and a single will, and is of but one substance; all three persons are coeternal, none of them having been created by another. However, because God exists in three persons, God has always loved, and there has always existed perfectly harmonious communion between the three persons of the Trinity. One consequence of this teaching is that God could not have created Man in order to have someone to talk to or to love: God “already” enjoyed personal communion; being perfect, He did not create Man because of any lack or inadequacy He had. Thus we find God saying in Genesis, “Let us make man in our image”.
One illustration employed by the early Fathers was to look at the sun, the sun’s rays, and the sun’s warmth. Another is the flame from three burning sticks, held so that their ends are together; each stick is a flame, but the difference between the flames of one stick and another cannot be seen.
However, any attempt to explain this mystery quickly breaks down, and all are of limited usefulness. The difference in thinking between those who believe in the Trinity, and those who do not, is not an issue of understanding the mystery. Rather, the difference is one of belief concerning the personal identity of Christ. It is a difference in conception of the salvation connected with Christ, that drives all reactions, either favorable or unfavorable, to the doctrine of the Holy Trinity. As it is, the doctrine of the Trinity is directly an issue of Christology.
A rather difficult but useful explanation of the interrelationship of the separate persons of God is called perichoresis, which means, envelopment (taken woodenly the Gk says, “dancing around”). This concept refers for its basis, to the Gospel of John, 14-17, where Jesus is instructing the disciples concerning the meaning of his departure. His going to the Father, he says, is for their sake; so that he might come to them when the “other comforter” is given to them. At that time, he says, his disciples will dwell in him, as he dwells in the Father, and the Father dwells in him, and the Father will dwell in them. This is so, according to the theory of perichoresis, because the persons of the Trinity “reciprocally contain one another, so that one permanently envelopes and is permanently enveloped by, the other whom he yet envelopes.” (Hilary, Concerning the Trinity, 3:1).
This co-indwelling may be helpful in illustrating the trinitarian conception of salvation. The first doctrinal benefit is that it effectively excludes the idea that God has parts. Trinitarians affirm that God is a simple, not an aggregate, being. God is not parcelled out into three portions. The second doctrinal benefit, is that it harmonizes well with the doctrine that, the Christian’s union with the Son in his humanity brings him into union with one who contains in himself the fullness (not a part) of deity (See also: Theosis). Perichoresis provides an intuitive figure of what this might mean. The Son, the eternal Word, is from all eternity the dwelling place of God; he is, himself, the “Father’s house”, just as the Son dwells in the Father and the Spirit; so that, when the Spirit is “given”, then it happens as Jesus said, “I will not leave you as orphans; for I will come to you”.
Eternal generation and procession
Some of the most problematic language in trinitarianism, is the affirmation that the Son is “begotten” and the Spirit “proceeds”, but the Father is “neither begotten nor proceeding”. The difficulties of this language are evident in the timefulness of the terms, where no beginning or process in time is intended; the language is instead meant to indicate the relationships among the persons of God. The implied analogy in the eternal being of God, with timeful events, apparently breaks down very quickly, so that the Christian churches have been frequently troubled on its account. Nevertheless, it is the language employed by all branches of Trinitarian Christianity. These concepts were the catalyst of the Great Schism, for example, concerning the Filioque clause.
These terms are employed in an effort to expand upon the economical subordination, implied by the genitive of terms like “Father of”, “Son of”, and “Spirit of”. While orthodox trinitarianism rejects ontological subordination, it affirms that the Father has a monarchial relation to which the Son and Spirit are subject. Or, in other terms, it is from the Father that the mission of the Breath and Word originate: whatever God does, it is the Father that does it, and always through the Son, by the Spirit. The Father is seen as the “source” or “fountainhead” from which the Son is begotten and the Spirit proceeds, much as one might observe water bubbling out of a spring without worrying about when it began doing so. However, this language is hemmed in with qualifications so severe that the analogy in view is easily lost, and is a source of perpetual controversy.
Nevertheless, the concept is considered to be of momentous practical importance to the Christian life because, again, it points to the nature of the Christian’s reconciliation with God. The excruciatingly fine distinctions can issue in grand differences of emphasis in worship and government, as large as the difference between East and West, which for generations now have been considered practically insurmountable.
Orthodox, Catholic and Protestant distinctions
The Western (Catholic) tradition is less timid about positive statements concerning the inter-relationship of persons in the Trinity. It should be noted that explanations of the Trinity are not the same thing as the doctrine itself; nevertheless the Augustinian west is inclined to think in philosophical terms concerning the rationality of God’s being, and is prone on this basis to be more open than the East, to seek formulations which make the doctrine more intelligible.
For example, one explanation is based on deductive assumptions of logical necessity: which hold that God is necessarily a Trinity. On this view, the Son is the Father’s perfect conception of his own self. Since existence is among the Father’s perfections, his self-conception must also exist. Since the Father is one, there can be but one perfect self-conception: the Son. Thus the Son is begotten by the Father in an act of intellectual generation. By contrast, the Holy Spirit proceeds from the perfect love that exists between the Father and the Son: and as in the case of the Son, this love must share the perfection of real existence. Therefore, as reflected in the filioque clause inserted into the Nicene Creed by the Catholic Church, the Holy Spirit is said to proceed from both the Father “and the Son.” The Eastern Orthodox church holds that the filioque clause, i.e., the added words “and the Son” (in Latin, filioque), constitutes heresy. One reason for this is that it undermines the personhood of the Holy Spirit; is there not also perfect love between the Father and the Holy Spirit, and if so, would this love not also share the perfection of real existence? At this rate, there were would be an infinite number of persons of the Godhead, unless some persons were subordinate so that their love were less perfect and therefore need not share the perfection of real existence.
Most Protestant groups that use the creed also include the filioque clause. However, the issue is usually not controversial among them because, their conception is generally less exact than is discussed above. The clause is often understood by Protestants to mean that the Spirit is sent from the Father, by the Son – a conception which is not controversial in Catholicism or Orthodoxy, either. Protestantism is harder to describe however, because of its lack of a unified tradition. The Protestant religious climate, which generally eschews any appeal to tradition, makes it more likely that rejected alternatives to Trinitarianism will be revisited. In some cases these alternatives have been formally adopted, which the Catholic and Orthodox churches have rejected as heresies, including a practical tri-theism (the distinction of persons implies a distinction in being), Nestorianism (a distinction in Christ’s natures implies a distinction in persons), Sabellianism (or Modalism, in which each person is merely another mode or role played by the same God), and Arianism (hero-adoration of Jesus, as opposed to religious worship of God alone, and of Christ as God incarnate, and of the Spirit as the presence of God within the believer), etc. In those cases where such alternatives are formally adopted, as opposed to being mistakenly substituted for orthodoxy, Protestantism drops identification with those groups, in effect upholding the trinitarian tradition as a biblical doctrine.
Because Christianity converts cultures from within, the doctrinal formulas as they have developed bear the marks of the ages through which the church has passed. The rhetorical tools of secular philosophy, especially of Neoplatonism, are evident in the language adopted to explain the church’s rejection of Arianism, Sabellianism, , etc. Augustine of Hippo has been noted at the forefront of these formulations; and he contributed much to the speculative development of the doctrine of the Trinity as it is known today, in the West; the Cappadocian Fathers (John Chrysostom, Basil the Great, and Gregory Nazianzus) are more prominent in the East. The imprint of Augustinianism is found, for example, in the western Athanasian Creed, which, although it bears the name and reproduces the views of the fourth century opponent of Arianism, was probably written much later.
These controversies were for most purposes settled at the Ecumenical councils, whose creeds affirm the doctrine of the Trinity. Constantine the Great who called the first of these councils, in AD 325, arguably had political motives for settling the issue rather than religious reasons; as he personally favored the Arian party, which in politically key regions of the Empire held a majority over the Catholics. It was also the form of Christianity that had been adopted by northern tribes of Vandals, and it would have given Constantine an advantage in defense against them, if the council adopted the same faith. It was not to be. The arguments of the deacon Athanasius prevailed; and over the next three hundred years, the Arians were gradually converted to Catholicism.
According to the Athanasian Creed, each of these three divine Persons are said to be eternal, each said to be almighty, none greater or less than another, each said to be God, and yet together being but one God. According to the teachings of orthodox Christianity, the three persons of the Holy Trinity are said to share one Divine Nature, thus preserving their belief in one God. So are we forbidden by the catholic religion to say; There are three Gods or three Lords. — Athanasian Creed, line 20
Some opponents of this view contend that these three “Persons” are not separate and distinct individuals. The modalists attempted to resolve the mystery of the Trinity by holding that, while God is numerically one, the Father, the Son and the Holy Ghost are merely modes, roles, or manifestations of God Almighty. These titles describe how humanity has interacted with or had experiences with God. In the Role of The Father, God is the provider and creator of all. In the mode of The Son, man experiences God in the flesh, as a human, fully man and fully God. God manifests Himself as the Holy Spirit by his actions on Earth and within the lives of Christians. This view is known as Sabellianism, and was rejected as heresy by the Ecumenical Councils although it is still prevelant today among denominationss known as “Oneness” Christians. Trinitarianism insists that the Father, Son and Spirit simultaneously exist, each fully the same God.
Some feminist theologians refer to the persons of the Holy Trinity with more gender-neutral language, such as Creator, Redeemer, and Sustainer (or Sanctifier). This is a very recent formulation, and emphasizes their roles rather than their personhood. Since, however, each of the three divine persons participates in the acts of creation, redemption, and sustaining, traditional Christians reject this formulation as simply a new variety of Modalism.
The doctrine developed into its present form, precisely through this kind of confrontation with alternatives; and the process of refinement continues in the same way. Even now, ecumenical dialogue between Eastern Orthodox, Oriental Orthodox, Roman Catholic, and trinitarian Protestants, seeks an expression of trinitarian doctrine which will overcome the extremely subtle differences that divide them into separate communions. The doctrine of the Trinity is symbolic, somewhat paradoxically, of both division and unity.
Dissent from the doctrine
Many Christians believe that the orthodox doctrine of the Trinity is so central to the Christian faith, that to deny it is to reject the Christian faith entirely. However there have been a number of groups both historical and current which identify themselves as Christians but yet have an alternative view of the trinity. One ancient sect, called Ebionism, said that Jesus was not a “Son of God,” but rather an ordinary man who was a prophet — a view of Jesus shared by Islam. Other groups have an understanding of the Trinity that differs from orthodox formulation shared by Catholics, Protestants, and Eastern Orthodox. These include the Church of Jesus Christ of Latter-day Saints, the Jehovah’s Witnesses (who reject the Trinity doctrine entirely), Christian Science, the Unification Church, Unitarian Universalists and Oneness Pentecostals (Who believe that there is one God with no essential divisions in His nature. He is not a plurality of persons, but He does have a plurality of manifestations, roles, titles, attributes, or relationships to man. Furthermore, these are not limited to three).