Years ago, when I was a family therapist in the counseling ministry of the local megachurch, a young couple recounted a hurtful, destructive argument that occurred when the young bride asked to put a “chair” in her husband’s office so she could be near him when he worked at home. Because his office was quite small, the young husband was irate and annoyed because there simply was not enough room for another “chair.” The young wife felt hurt and rejected because she thought her husband did not want her near when he was working. As the couple disclosed their feelings in counseling, the wife revealed that she had merely wanted to put a small straight-back chair in a tiny corner of the room, where she could read as her husband worked. With some embarrassment, the young husband admitted that he thought she wanted to bring in a large “easy ‘chair’” from the living room, one that would take up far too much space in an already overcrowded room. Even though they were using the same word, the couple had argued because they attached very different meanings to the word “chair.”
Language matters; words are important. Moreover, the meaning attached to words is crucial if confusion and misunderstanding are to be avoided. Perhaps nowhere is language more problematic and the meaning of words more subject to misunderstanding than in the doctrine of the Trinity—the belief that the One God of the Christian faith eternally exists as three persons: Father, Son and Holy Spirit.
Recently, as I quickly flipped through the surplus of “Christian” channels invading my home via satellite, I stopped at an Australian broadcast when I heard the word “Trinity.” The host was asking her guest, an “expert” on the doctrine,” “How can God be one and three? How can one ‘equal’ three? she asked. “The ‘math’ just doesn’t add up,” she said. Her questions betray the common misunderstanding of the doctrine of the Trinity as a “mathematical” puzzle.
A few years ago, a survey was taken among a group of church members in London, who were asked, “How can God be three persons in one?” Showing their misunderstanding of the “oneness” of God, about one-third of the respondents replied that God was “one” in the sense of being “one person.” As one respondent typically affirmed, “The three are one person; they’ re all one person.”
To be sure, much confusion exists regarding the doctrine of the Trinity. The confusion is exacerbated by preachers who describe the doctrine of the Trinity as a mind-boggling mystery or an incomprehensible enigma far beyond the limits of human understanding. While it is true that we finite humans are incapable of fully comprehending the infinite God, it is not true, however, that the doctrine of the Trinity is beyond our understanding. A “doctrine” is simply an attempt to put into words what we do know about God based upon God’s self-revelation of himself in the history of salvation. The “doctrine of the Trinity” is an attempt to make sense of the fact that the “one God” of the Christian faith has revealed himself in the Holy Bible in “three persons”—Father, Son and Holy Spirit. The biblical narrative of the Father’s reconciliation of the world in Jesus Christ, as well as his bringing that work to fruition by the Spirit, implies a Trinitarian understanding of God (see 2Cor 5:18-20; Rom 5:1-5; Eph 1:3-14).
The early Church was composed, at least initially, of Jews. In distinction to the cultures around them who worshipped many gods, the Jews worshipped one God. At the same time, the early Jewish Christians believed that God had come in the flesh and dwelt among them in the person of Jesus Christ (John 1:1, 14). They believed that Jesus is “Immanuel”: God with us (Matt 1:23). Moreover, the early Christians believed that the crucified Christ remained present among them through the Holy Spirit (2Cor 3:17, 18).
The believers of the early Church, many of whom were slaves who could neither read nor write, did not concern themselves with abstract speculation about the nature of God; yet, their worship and practice was distinctly Trinitarian in character. Following the commandment of Jesus Christ (Matt 28:19), the early Church baptized in the name of the Father, Son and the Holy Spirit, even as they declared the love of God, the grace of Jesus Christ and the communion of the Holy Spirit (2Cor 13:14). Early second-century writings—including the 1) Didache, an early writing on Church order and practice; 2) Hippolytus’ Holy Communion prayer and baptismal formula, and 3) Justin Martyr’s early description of a Christian worship service and baptism—portray Christians baptizing and celebrating Holy Communion (or the Lord’s Supper) in the name of the Father, Son and Holy Spirit.
Development of the Doctrine of the Trinity
From the “raw material” provided in the worship and practice of the early Church, Christian theologians began their construction of the “doctrine” of the Trinity. The starting point for Christian reflection on the nature of God is the relationship between God and Jesus Christ. The problem faced by early Christian theologians, as they pondered the New Testament witness to Jesus Christ,  was not whether Jesus was God, but how, within the boundaries of their inherited monotheism, could they maintain the unity of God while confessing the deity of one who is distinct from God the Father. That is, how could the early Church claim that Jesus is one with God while maintaining there is only one God?
As the early Church began to proclaim the deity of Christ, they encountered opposition from those who distorted the New Testament witness to the Triune nature of God. In the second century, some incorrectly argued that the terms “Father,” Son” and “Holy Spirit” are merely different “names” for God, each designating a different “role” played by a “one-person” God, like a single individual who plays the roles of spouse, employee and soccer coach on a given day. Others wrongly argued that the Father, Son and Holy Spirit are distinct “individuals,” like the coach, quarterback and wide receiver on a football team. The first error, known historically as modalism, preserves the one “being” of God but loses the specific identity of the three persons of the Godhead by reducing the Father, Son and Spirit to one person. The second error, tritheism (or “pluralism”), stresses the “distinction” of the three persons of the Godhead at the expense of the “unity” of God and results in “three gods,” rather than “one God in three persons.” Quite importantly, both errors fail to express the essential Trinitarian element of relationship among the three persons of the Godhead. The first precludes relationship by reducing the Father, Son and Holy Spirit to “one person.” Less obviously, the second error precludes relationship, for though the three persons may function together in a limited way, they are not “one” in terms of sharing a common “being.”
In the face of these distortions of the New Testament witness to the nature of God, early Christian thinkers struggled to accurately express God’s triadic self-revelation in salvation history as Father, Son and Holy Spirit, while stringently maintaining the unity of the one God of the Judeo-Christian heritage.
Justin Martyr, the great “apologist” who defended the early second-century Church against false charges brought against Christians, invoked the image of light to capture the eternal relation between the Father and the Son. Justin captured both the equality and the distinction of the Father-Son relation by arguing that the Son is indivisible from the Father in the same way that light emitted by the sun is indivisible from its source. His metaphor became a favorite among the Church fathers and was later enshrined in the Nicene-Constantinopolitan Creed, where one of several phrases used to describe Jesus Christ is “Light from Light.”
Irenaeus, an important theologian of the second-century, developed his Trinitarian insights in contention with the Gnostics, who erroneously thought of God as utterly transcendent and completely separate from the taint of the “evil” material world. To the contrary, Irenaeus argued that God the Father interacts with creation through his “two hands,” that is, the Son and the Spirit. For Irenaeus, the Son and Spirit belong intrinsically and eternally to the being of God, as the hands of a sculptor belong intrinsically to the artist and are the means of his or her creative expression.
In the third century, the North African lawyer Tertullian coined the word “Trinity” (Latin: trinitas) and argued that Christians worship “one God in three persons.” For Tertullian, “being” or “nature” is the unifying principle of the Godhead, that is, what the three persons of the Trinity have in “common.” “Person” is the principle of “distinction” or “otherness”; that is, the Father is not the Son, the Son is not the Father, and the Father and Son are not the Holy Spirit. Rather, each person of the Triune God is “distinct” from the other.
The fourth century was a time of great conflict among the theologians of the early Church. Since the time of Tertullian, confusion had existed between the Greek-speaking theologians of the eastern Mediterranean and the Latin-speaking theologians of the west regarding the proper translation of important Trinitarian terms such as “being” and “person.” To add to the confusion, these terms were often used interchangeably, much as today when a single individual may be described both as a “person” and as a human “being.” Prior to the fourth century, the universal Church simply lacked the conceptual and linguistic resources to express how God is both one and three.
This confusion in terminology climaxed in one of the greatest theological conflicts in the history of the Church. Arius, a deacon from Alexandria, argued that the “one being” of God cannot be “divided,” for such would result in more than one God and compromise the inviolate principle of monotheism. For Arius, therefore, Jesus Christ cannot participate in the “being” of God; that is, he is not fully divine; rather, he is “subordinate” in being to God. Much like modern-day Jehovah’s Witnesses, Arius argued that Jesus is a created being, that is, an exalted “creature,” like an archangel, who is less than fully God. The great Athanasius, one of the most important theologians in the history of the Church, stalwartly defended the deity of Jesus Christ against the subordinationism of Arius. As Athanasius understood, if Jesus is a “created” being, he cannot be the “eternal” Word of God “incarnate,” that is, God in human flesh (John 1:1, 14). For Athanasius, this was no mere academic theological squabble; to be sure, nothing less than human salvation was at stake, for if Jesus Christ is not fully God, then we are still in our sins, for only God can save.
In what has been called the most important theological statement since the New Testament, Athanasius argued that the incarnate Son of God, Jesus Christ, is “of one being with the Father.” That is, Jesus Christ is fully God, just as the Father is God. Athanasius’ defense of the full deity of Jesus Christ was enshrined in the Nicene-Constantinopolitan Creed (381 A.D.), where, in accordance with the apostolic witness recorded in the New Testament, the Church fathers declared that Jesus Christ is “God of God, Light of Light, Very God of Very God, Begotten, not made, Of one being with the Father.” At the same time, the fathers asserted the full deity of the Holy Spirit. 
With the assertion of the full deity of Jesus Christ and the Holy Spirit against distortions of the New Testament witness to the triadic nature of God, the way was cleared in the late fourth century for the classic, orthodox statement of the Christian doctrine of the Trinity, formulated by a trio of theologians—Basil the Great, Gregory of Nyssa and Gregory Nazianzus—known collectively in Church history as the “Cappadocian Fathers.” By precisely defining important Trinitarian terms such as “being” and “person,” these Greek-speaking theologians were able to conceptually express the unity (“one-ness”) and diversity (“three-ness”) of the Triune Godhead in a way similar to that of the Latin theologian Tertullian of a century earlier. In view of the triadic pattern of God’s self-revelation in salvation history as Father, Son and Holy Spirit, the Cappadocians argued that God exists as “one being” (i.e., “nature,” “essence”) in “three persons,” where “being” is the principle of unity and “person” is the principle of distinction or diversity. As the Cappadocians argued, the divine persons of the Trinity share a common “being”; at the same time, they are three distinct “persons.” In other words, “what” Father, Son and Spirit are is the same; “who” each is is distinct and unique.
It is important to note that the terms “being” and “person,” as used by the Cappadocian fathers, are not interchangeable. If we say God is “three beings,” we commit the error of “tri-theism.” If we say God is “one person,” we commit the error of “modalism.” The Cappadocian formula—“one being, three persons”—preserves both the “unity” (one-ness) and the “diversity” (three-ness) of the Godhead, while articulating the Trinitarian grammar that would allow the Church to speak of God as “one being in three persons”—One in Three, Three in One.
In addition, it is essential to note that the Father, Son and Spirit cannot be thought of as independent, autonomous “selves,” as the modern use of the term “person” suggests. For Athanasius and the Cappadocian fathers, the term “person” inherently includes relationship, for the terms “Father” and “Son” are necessarily relational. There can be no “Father” apart from the “Son”; there can be no “Son” apart from the “Father.” Thus, the divine persons in relationship constitute the “being” of God. At the same time, each divine person is unique in terms of “origin.” In Trinitarian language, the Father is “un-begotten,” the Son is “begotten” and the Spirit “proceeds.” The language of relationship captures the “unity” of the persons of the Holy Trinity, while the language of origin captures the distinctiveness or “diversity” of the divine persons.
Finally, at the heart of the Holy Trinity, the Cappadocians saw an interpersonal communion (koinonia) or “fellowship,” where each divine person is intimately related to the other two in reciprocal joy and delight. The internal relatedness of the divine persons is expressed in the Trinitarian concept, perichoresis (Latin: “coinherence”), where the divine persons are said to mutually “indwell” and permeate one another in a divine “dance” of intimate fellowship and communion.
Putting all this together, we can say that the Father, Son and Holy Spirit, mutually indwelling one another in an intimate communion of love, is the “one God” of the Christian faith. As the Cappadocian father Gregory Nazianzus put it, “When I say ‘God’, I mean the Father, the Son and the Holy Spirit.” The Cappadocian formula—“one being, three persons—with its regard for the importance of “relationship” as an integral aspect of the “being” of the one God, is enjoying renewed appreciation today among leading contemporary Trinitarian theologians.
God is Love
In light of what may appear to be theological hair-splitting about the nature of God, does the doctrine of the Trinity really matter? Does it make any difference whether God is “one being in three persons,” as the doctrine of the Trinity asserts, or simply one person who plays three roles (i.e., “modalism”) or even three different gods pursuing their own ends (i.e., “tritheism”)? In regard to the last point, if Christians are really polytheists who worship three “gods,” rather than “three persons in one God,” then we can never be certain that God is like Jesus. The Church fathers’ assertion that Jesus is “of one being with the Father” concisely expresses the biblical truth that the loving heart of Jesus is a window into the inner heart of the Holy Trinity. The unity of “being,” as well as the unity of will and purpose between the Father and the incarnate Son (John 5:30) assure us that there is no dark, inscrutable god hidden behind the back of Jesus Christ, but only the God who has loved us to the uttermost in sending his Son to be our Savior. Thus, it matters whether the Holy Trinity is three “gods,” each independently seeking his own ends, or “one God in three persons,” who enjoy unity of being, harmony of will and singleness of purpose in creating humanity to share in the life and love of the Father, Son and Holy Spirit.
Moreover, if God is only “one person” who plays three different “roles, then the apostolic witness to the nature of God is called into question. According to the apostle John, “God is love” (1John 4:8, 16). For John, love is not one characteristic among many that we “attribute” to God; rather, God is love. Yet, what is godly love like? In his memorable treatise on love (see 1Cor 13), the apostle Paul writes that love is patient and kind. It does not envy or dishonor others. Love is not self-seeking. It keeps no record of wrongs. Note that Paul describes love in interpersonal terms; that is, he describes love in terms of relationship. To be sure, godly love is relational, for by its very nature, love requires another.
In regard to the doctrine of the Trinity, “Father” and “Son” are terms of relationship. God is not an “in-itself,” apart from others, but is “the epitome of love in relation.” God is not alone, in isolation from relationships, but is eternally related within the Holy Trinity as Father, Son and Holy Spirit. The one God of the Christian faith eternally exists in a Triune communion of relationship whose nature is “love”: the Father loves the Son in the Holy Spirit; the Son loves the Father in the Holy Spirit.
On the other hand, if God is unitarian rather than trinitarian, that is, “one” divine person who plays three different “roles,” then God cannot be eternally love; rather, God becomes love when he creates another. In that case, we cannot be certain of God’s purpose in creation, for a one-person god isolated in eternal “alone-ness” may create from a need for fellowship. If so, then creation is not God's free and gracious act for us but is, rather, a self-fulfilling act designed to fill the one-person-god’s need for community.
Happily, because scripture reveals that God is a divine communion of love, eternally existing as Father, Son and Holy Spirit, we can be certain that there is no lack or necessity in God. God did not create us to fill a void or need in the Godhead; rather, God created us as an act of overflowing love, for by its nature godly love cannot be contained; it reaches out in self-giving for us. God created the world in order to share his divine life and love with all humanity. That is why we were born: to be included in the divine life and love of the Holy Trinity, to participate in and enjoy the eternal communion of fellowship shared by the Father, Son and Holy Spirit.
Eternally Father, Son and Holy Spirit
The doctrine of the Trinity is the Church’s attempt to express within the limitations of human thought and speech the biblical witness to the eternal nature of God whom the New Testament describes as “love.” God’s self-revelation in the history of salvation as Father, Son and Holy Spirit is finally God’s self-witness to his eternal, loving purpose for the whole world. Most importantly, the doctrine of the Trinity is the Christian assertion that God is antecedently and eternally the same God who has revealed himself in the history of human salvation as Father, Son and Holy Spirit. In short, we know who God is from what he does. There is no other God than the loving Father who has loved us to the uttermost in the sending of his Son and the gift of the Spirit—all for us and for our salvation. “May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all” (2Cor 13:14).
 In Fiddes, P.S. Participating in God: A Pastoral Doctrine of the Trinity. (Louisville, KY: Westminster John Knox Press, 2000), pp. 4-5.
 See, for example, John 1:1, 14; 10:30; 14:9; Col 1:16, 17; 2:9; Heb 1:3
 Regarding the divinity of the third person of the Godhead, Scripture describes the Holy Spirit as one who is a “personal, encountering, interacting Thou” clearly distinguishable from the Father and Son. The Spirit speaks in the first person (Acts 10:20; 13:12), teaches (Jn 14:26), stands as witness (Rom 8:16; 1Cor 1:22; 5:5; Eph 1:13, 14), sends (Acts 13:2), grieves (Eph 4:3), struggles with other persons (Gen 6:3; Isa 63:10) and gives gifts (1Cor 12:4-11; Eph 6). Moreover, the names (Acts 1:8; Jn 4:24; 14:21; 15:26; Rom 8:14), attributes (Heb 9:14; 1Cor 2:10-12; Lk 11:20; Rom 15:18-19) and works (1Cor 2:10; Acts 5:30-32; 28:25; Titus 3:5; 1Cor 6:19) of God are ascribed to the Spirit. See Oden, T.C. Systematic Theology, Volume One: The Living God. (Peabody, MA: Hendrickson, 2006), p. 199.
 As an aid to memory, we might say that, in the Holy Trinity, there is one “what” (“being”) and three “who’s” (“persons”).
 This phrase was commonly used by the great 20th century Scottish theologian, T.F. Torrance.
 Sanders, J. The God Who Risks: A Theology of Divine Providence. (Downers Grove: IVP, 2007), p. 148.