Responsible Christian theology speaks of God on the basis of the particular acts of God attested in Scripture. The biblical basis for the tri-unity of God is found in the pattern of the revelatory acts of God throughout the biblical narrative.  Indeed the trinitarian idea emerged formally in the early centuries of the church as an explanation for the Christian belief in the one God who had also been revealed in the person of Jesus and who was experienced through the ministry of the Spirit. The basic premise of Trinitarian theology is summarized by Daniel Migliore:

If God is expressed to us in three distinct personal ways, then there is a basis of this structure of divine love in God's own immanent, eternal being.  God's own life cannot contradict what God is in relation to the world.  In God's own life there is an activity of mutual self-giving, a community of sharing, a "society of love" (Augustine) that is the basis of God's history of love for the world narrated in Scripture.[1]

In this sense, the message of the gospel is the assurance that God's identity is the same as was manifested by the Son - God is love.  Negatively, the danger of all heresy lies in so misconstruing the identity of God that the gospel is invalidated. 

In order to make sense of the history of God's revelatory acts Christians had to develop a concept of God - and the concept that is coherent with His acts is, distinctly, Trinity.  To this end, "The logic of Trinitarian theology moves from the differentiated love of Father, Son, and Holy Spirit in the economy of salvation (the economic Trinity) to the ultimate ground of this threefold love in the depths of the divine being (the immanent Trinity)."[2]  In other words, responsible Trinitarian thinking moves from confession to doctrine, from the revelatory acts of God to the essential being of God. 

Historically, the formal doctrine of the Trinity emerged in the midst of several controversies over the concept of God.  In the third century, defending itself against Arianism, the church was concerned with asserting the identical - rather than merely generic - unity of the essence of God.  Fifty years later, the emphasis on one essence encountered the threat of Sabellianism, so the attention of the church turned to the distinctions within the one essence.[3]  The first issue was settled at the Council of Nicea.  After the shift of concerns, the Cappadocian fathers - Basil the Great, Gregory of Nyssa, and Gregory of Nazianus - brought the developments of the idea of God to their climax at the reaffirmation of the Nicene creed, and the inauguration of the complete doctrine of Trinity, at the Council of Constantinople in 381.[4]

The now-creedal formulations of Trinity grew up through the theological remolding of philosophical terms (and, importantly, not the other way around).  In defense of the substantial unity of God, Athanasius posited homoousios (‘of same substance') over against homoiousios (‘of like substance') over and against Arianism.  The angle of approach for the Cappadocians was different from Athanasius in that they made the three hypostases (Persons) their starting point.  In this way, the eastern attempts to work out an understanding of God stem from the attempt to make sense of the plural experience of God in the context of the belief that God is one.  The Cappadocians' great achievement, philosophically, was the elevation of hypostasis as the ultimate ontological category, rather than substance.[5]  They did this by defining the distinct hypostases in tension with common ousia in the context of communion.[6]

That the Godhead can exist "undivided...in divided Persons"[7] is made possible by the doctrine of the co-inherence, or the perichoresis, of the divine Persons.  The importance of this affirmation that God exists in communion is that "every self-communication presupposes the capacity for self-differentiation... an individuality cannot communicate itself."[8]  The importance of this cannot be overstated, since the ability of the Trinity to be completely self-communicated in each of the Persons is key to explaining the three centers of divine activity in the history of God's revelatory acts.  In this way, Trinity redescribes God in light of the event of Jesus Christ and the outpouring of the Spirit.  The doctrine of the Trinity reached by the Cappadocians demonstrates that "God is self-expending, other-affirming, community-building love."[9]  God is what he says and does in the gospel. 

The twentieth century renaissance of Trinitarian theology has included several theologians showing interest in this eastern approach to the Trinity.  Two of these eastern-leaning scholars are Jurgen Moltmann and T.R. Martland.  In The Trinity and the Kingdom of God, Moltmann explains that for Augustine and Aquinas the "one, common, divine substance counted as being the foundation of the trinitarian Persons and was hence logically primary in comparison."[10]  He suggests that taking the philosophical claim as the starting point leads to serious consequences, namely an unfortunate distinction between ‘the God one' and ‘the God many.'  He says:

The result is that the first unity forces out the second.  Consequently, not only is there undue stress on the unity of the triune God, but there is also a reduction of the tri-unity to the One God.  The representation of the trinitarian Persons in a homogenous divine substance, presupposed and recognizable from the cosmos, leads unintentionally but inescapably to the disintigration of the doctrine of the Trinity in abstract monotheism.[11]

In an article about the methodology of Augustine and the Cappadocians, Martland is even more incisive in his accusations of Augustine:

The doctrine which the Cappadocians understood to be an explanation for the presence of the encountered deity in history, because of the Augustinian emphasis upon the philosophical premise of divine unity, now becomes a dogma with no reason for its acceptance other than its being a mark of orthodoxy.[12]

Both agree, in terms of logic, that "it seems to make more sense theologically to start from the biblical history, and therefore to make the unity of the three divine Persons the problem, rather than...to start from the philosophical postulate of absolute unity, in order then to find the problem in the biblical testimony."[13]   It follows that the concept of God's unity must be perceived in the communion of the divine Persons in order to demonstrate a consonance between God's life and the biblical testimony of God's love enacted in the Son by the Spirit.

The point bears repeating that a reliable and meaningful understanding of the Trinity begins with confession of God's revelatory acts and only then moves to assert a doctrine of his essential being.   Rightly understood, the doctrine of the Trinity demonstrates that God is what he says and does in the gospel.  To that end, the Cappadocian Fathers were right to start from the biblical history of God represented in three distinct Persons and then seek to understand His unity in a way that affirmed the integrity of His redemptive acts with his own Being.  And the contemporary emphasis on eastern thought in Trinitarian theology is good, in that it emerges out of similar concerns - for a confession of the triune God that remains firmly rooted in Scripture and a doctrine that contributes to meaningful orthodoxy. 

Bibliography

Green, Brad. "The Protomodern Augustine?  Colin Gunton and the Failure of Augustine," International Journal of Systematic Theology 9 (July 2007). 

Gunton, Colin. "Being and Person: T.F. Torrence's Doctrine of God" The Promise of Trinitarian Theology.  New York: Rowman and Littlefield, 2001: 115-135. 

Hastings, Ross.  "God Three and God One: The Cappadocians and Trinitarian Theology" unpublished lecture, 2007. 

Kelly, J.N.D.  Early Christian Doctines.  London: Adam and Charles Black, 1958.

Martland, T.R. "A Study of Cappadocian and Augustinian Methodology" Anglican Theological Review 47 (J1 1965): 252-263.

Migliore, Daniel.  Faith Seeking Understanding: An Introduction to Christian Theology. Grand Rapids: Eerdmans, 1991.

Moltmann, Jurgen.  The Trinity and the Kingdom.  Minneapolis: Fortress Press, 1993.

Plantinga, Cornelius, Jr. "Social Trinity and Tritheism," Trinity, Incarnation, and Atonement.Notre Dame, IN: University of Notre Dame Press, 1989: 21-47.

[1] Daniel Migliore, Faith Seeking Understanding: An Introduction to Christian Theology (Grand Rapids: Eerdmans, 1991), 61.

[2] Migliore, Faith Seeking Understanding, 62.

[3] T.R. Martland, "A Study of Cappadocian and Augustinian Methodology" Anglican Theological Review 47 (J1 1965), 253.

[4] J.N.D. Kelly, Early Christian Doctines (London: Adam and Charles Black, 1958), 263.

[5] Ross Hastings, "God Three and God One: The Cappadocians and Trinitarian Theology" (unpublished lecture).

[6] see Kelly, Early Christian, 263-269.

[7] Gregory of Nazius, cited in Kelly, Early Christian, 264.

[8] Jurgen Moltmann, The Trinity and the Kingdom, (Minneapolis: Fortress Press, 1993), 57.

[9] Migliore, Faith Seeking Understanding, 63.

[10] Moltmann, The Trinity, 16.

[11] Ibid., 16-17.

[12] Martland, "Cappadocian and Augustinian," 257.  For other arguments for the eastern perspective, see Colin Gunton, "Being and Person: T.F. Torrence's Doctrine of God," The Promise of Trinitarian Theology (New York: Rowman and Littlefield, 2001), 115-135.  Brad Green, ‘The Protomodern Augustine?  Colin Gunton and the Failure of Augustine,' International Journal of Systematic Theology 9 (No. 3), July 2007.  Cornelius Plantinga, Jr., "Social Trinity and Tritheism," Trinity, Incarnation, and Atonement (Notre Dame, IN: University of Notre Dame Press, 1989), 21-47.

[13] Moltmann, The Trinity, 149.