One of the more common words utilized by the apostle Paul to describe both the character and activity of the God made known in Jesus is mystery. In other words, Christian Theology is at some level both unexplainable and beyond definition. This neither means that theology is pursued in vain, nor that any attempt to speak of God is void of truth. Instead, the maintenance of mystery within the discipline of Christian theology reminds the Christian theologian and Christian Church that all theology is modest theology  that humbly bears witness to the nature, character, and activity of God. Dr. Daniel L. Migliore  writes:
Christian theology begins, continues, and ends with the inexhaustible mystery of God. It speaks of God, however, not in vague and general terms, but on the basis of the particular actions of God attested in Scripture (Migliore 64).That being said, throughout history the primary means that Christian Theology has illustrated and confessed the mystery of God in Scripture is through trinitarian language, i.e. the self-revelation of God as Father, Son, and Holy Spirit. It could even be said that all Christian Theology (and Reformed Theology) is uniquely Trinitarian Theology that bears witness to God who is three-in-one.
Trinitarian Theology is difficult to explain and even more difficult to defend, as nowhere in Scripture is there a clear and clever description of the doctrine. That is to say, the doctrine of the trinity is not one directly revealed to us through Scriptural proof, but a doctrine interpreted, developed, and experienced by the faith community from the first generation of Christians up to the present day. Karl Barth affirmed the doctrine of the Trinity as the “work of the Church, a record of its understanding of the statement of its object [who is God]…regarded only as an interpretation” (Church Dogmatics Vol I.1 p. 308). Trinitarian Theology surfaced as God’s people interpreted and experienced the three-fold activity of the God of the Hebrew Scriptures (i.e. Old Testament), incarnated in the person and work of Jesus (as illustrated in the New Testament), and continually revealing God’s self and activity through the Holy Spirit. However, it has been essential for Orthodox Christian Theology to maintain not only the distinct yet unified manifestations of God (three-in-one), but also the eternal existence of God as Trinity, i.e. there never was a time when God was not trinity.
While much ink has been spilled over the precise language and the proper discourse used to explain, defend, and promote Trinitarian Theology, the doctrine must extend beyond mere philosophical jargon. That is to say, Reformed Christian Theology as Trinitarian Theology also provides the platform for a Missional Theology rooted in the missional nature and character of God. German Theologian, Jürgen Moltmann says it best, “We don’t believe in the Trinity only. We live in the Trinity; we live in God…surrounded from all sides.” Migliore offers yet another advantageous insight:
Trinitarian doctrine describes God in terms of shared life and love rather than in terms of domineering power. God loves in freedom, lives in communion, and wills creatures to live in a new community of mutual love and service. God is self-sharing, other-regarding, community-forming love (73).The reality of the trinity exposes the communal nature, redemptive activity, and sending spirit of the God revealed in the biblical witness and confessed by believing communities throughout history. In essence, common to Scripture and historical confessions, as God the Father sent the Son, the Father and Son sent the Spirit, so too has the Father, Son, and Holy Spirit sent God’s people to bear witness with their lives and lips to the good news of salvation that is for the whole world.
 Karl Barth wrote, “Evangelical theology is modest theology, because it is determined to be so by its object, that is, by him [God as trinity] who is its subject” (Evangelical Theology 7).
 All Migliore citations are from Faith Seeking Understanding (Eerdmans, 2004).
Helpful Quotations from Significant Voices
“When Christians speak of God as eternally triune, they simply affirm that the love of God that is extended to the world in Jesus by the Holy Spirit is proper to God’s own eternal life in relationship.”
Daniel L. Migliore, Faith Seeking Understanding, p. 70
“God is the majestic creator of the heavens and the earth, the servant redeemer of a world gone astray, and the transforming Spirit who empowers new beginnings of human life and anticipatory realizations of a new heaven and a new earth.”
Daniel L. Migliore, Faith Seeking Understanding, p. 67
“The biblical witness to God’s revelation sets us face to face with the possibility of interpreting the one statement that ‘God reveals Himself as the Lord’ three times in different senses. This possibility is the biblical root of the doctrine of the Trinity.”
Karl Barth, Church Dogmatics Vol I.1, p. 376
“We don’t believe in the Trinity only. We live in the Trinity; we live in God…surrounded from all sides.”
Jürgen Moltmann, 2010 Emergent Theological Conversation