The Christian understanding of God has traditionally been defined in Trinitarian terms. The Trinity is the way in which we name God. We maybe monotheistic, but Judaism and Islam have a much more consistent and narrow understanding of monotheism. In fact, Islam could be seen as a Unitarian offshoot of Christianity. Consider the assertion of Robert Wright that more marginalized Christian sects, such as the Ebonite’s, could have been a source of Islam’s birth. That is, Muhammad, in developing a form of monotheism for Arabia, may have looked to non-Trinitarian forms of Christianity, as well as Judaism for insights, and then adapted them to his own needs. Thus, for Muhammad, Jesus is a prophet, Messiah, and even one born of a virgin, but not “son of God” – at least not in the way traditional Christians have understood that idea.
As the late Disciples historian Ronald Osborn notes, Disciples have been ambivalent about the doctrine of the Trinity from the beginning. He writes:
The Disciples regarded themselves as neither Trinitarian nor Unitarian. Alexander Campbell would not use the term Trinitarian because it did not appear in scripture. He even changed one line in the great Trinitarian hymn, “Holy, Holy, Holy,” so that instead of saying “God in three persons, blessed Trinity,” people would sing, “God over all, and blest eternally.
That being said, Christians are by and large Trinitarian, though not all approach the Trinity in the same way. The subject is usually approached from one of two ways – either God’s involvement in the history of salvation (economic Trinity) or the nature of God’s being (immanent or ontological).
Of course, before one addresses either question, we must address the question of whether the doctrine of the Trinity is even biblical. Arius, the great opponent of Trinitarianism insisted that it wasn’t, pointing to the lack of biblical support for the idea. But, in making this claim he also questioned the idea of Christ's full divinity. Others, have been a little more subtle in their questions than Arius, but they also have found the doctrine difficult to accept.
Alexander Campbell objected to what he called the "Calvinistic doctrine of the Trinity" because it "confounds things human and divine, and gives new ideas to bible terms unthought of by the inspired writers." One of the ideas that Campbell found especially vexing was the pre-existence of the Son of God, an idea required by most Trinitarian theologies. Campbell insisted, however, that "there was no Jesus . . . no Son of God, no Only Begotten, before the reign of Augustus Cesar. The relation that was before the Christian era was not that of a son and a father, terms which always imply disparity." Instead, Campbell thought of the relationship as simply between God and the Word of God. As Word of God, Campbell could affirm pre-existence, but not as son. Campbell also had great difficulty with Trinitarian vocabulary, much of which he thought was unbiblical. Yet in the end he affirms the idea of the Trinity, even if he had difficulties with it:
Paul and Peter indeed speak of the divine nature in the abstract, or of the divinity or godhead. These are the most abstract terms found in the Bible. Eternity and divinity are, however, equally abstract and almost equally rare in Holy Writ. Still they are necessarily found in the divine volume; because we must abstract nature from person before we can understand the remedial system. For the divine nature may be communicated or imparted in some sense; and, indeed, while it is essentially and necessarily singular, it is certainly plural in its personal manifestations. Hence we have the Father, Son, and Holy Spirit equally divine, though personally distinct from each other. We have in fact, but one God, on Lord, one Holy Spirit; yet these are equally possessed of one and the same divine nature.
All of this suggests that the doctrine of the Trinity isn’t an easily understood or explained concept.
We need to state up front that much of the vocabulary that undergirds Trinitarian doctrine is not found in the Bible, and that includes the word Trinity itself. Few verses actually speak in Trinitarian formulas, in fact, only two passages actually provide us with a Trinitarian formula: Matt. 28:19 and 2 Corinthians 13:13. However, there are many verses that would suggest the need for the doctrine of the Trinity to make sense of the biblical understanding of God. We will look at some of this today. Brevard Childs puts the issue in this way:
It is a formulation of the church in its attempt to reflect faithfully on the biblical witness. But it was precisely by observing the unity and differentiation of God within the biblical revelation that the church was confronted with the Trinity. The divine subject, predicate and object, are not only to be equated, but also differentiated. Indeed it is the doctrine of the Trinity which makes the doctrine of God actually Christian.
Ultimately, the need for a doctrine of the Trinity arose from the need to make sense of the church's affirmation of the divine sonship of Jesus Christ. As Brevard Child's points out the doctrine emerged from the need to "do justice to the Christ who was from the church's inception confessed as Lord." Child's also notes that when nineteenth century Christians lost interest in the doctrine of the Trinity their Christologies also began to blur and become distorted. 
If Osborn speaks out of the traditional Disciple reticence to define a Trinitarian viewpoint, a more recent Disciple theological discussion of the concept, suggests that Disciples need to develop a “robust Trinitarian theology.” Peter Goodwin Heltzel suggests that such an engagement is required of us because of our engagement in the ecumenical movement, but also because it allows us to recognize our relationality in diversity.
Although I recognize the ambivalence that is within my own religious tradition, I believe that Heltzel is correct that it would serve the Disciples well to engage more fully in developing a robust Trinitarian theology, one that affirms diversity and relationality.
Wright, The Evolution of God, pp. 361-363.
Ronald Osborn, The Faith We Affirm: Basic Beliefs of Disciples of Christ, (St. Louis: Chalice Press, 1979), p. 52.
Alexander Campbell, A Compend of Alexander Campbell's Theology, Royal Humbert, ed., (St. Louis: Bethany, 1961), pp. 94-98.
Alexander Campbell, The Christian System, (Cincinnati: H.S. Bosworth, 1866; reprint, Salem, NH: Ayer Company, 1988), p. 20.
Brevard Childs, Biblical Theology of the Old and New Testaments: Theological Reflection on the Christian Bible, (Minneapolis: Fortress Press, 1992), 375.
Childs, Biblical Theology, p. 376.
Peter Goodwin Heltzel, “Singing the Trinity,” in Chalice Introduction to Disciples Theology, (St. Louis: Chalice Press, 2008), pp. 92ff.