One of the most critical concepts in contemporary missiology is that of the missio Dei, the mission of God. Its importance is underscored by John Flett who writes:
Early in the twentieth century, many legitimate criticisms were being issued against the missionary enterprise. World War I and the loss of the claimed spiritual authority of Western civilization, the maturation of the so-called “younger” churches, the West’s own encounter with secularism and pluralism, the fierce reactions to colonialism, the growth of indigenous nationalist movements and related resistance of the non-Christian religions to Christian expansion – all challenged the right of cross-cultural cultural missions to exist. Against these criticisms, missio Dei supplied a theological redoubt for the missionary act by placing it within the Trinitarian being of God. This established a critical distance between “mission” and every contingent human form.
Following David Bosch’s now standard treatment, “mission is not primarily an activity of the church, but an attribute of God. God is a missionary God.” The Father sent his Son and Spirit into the world, and this act reveals his “sending” being. He remains active today in reconciling the world to himself and sends his community to participate in this mission…. The concept allowed theorists to acknowledge the legitimate charges laid against mission, while supplying an inviolate justification for the task itself. For Bosch, the importance of this “decisive shift,” as illustrated by its being “embraced by virtually all Christian persuasions,” cannot be doubted.
In other words, missio Dei is a critical component in our missiological thinking because it provides a firm biblical and theological foundation for the missionary endeavor. In an ever-increasingly relativistic and pluralistic world, it is crucial that the Christian message be undergirded by the conviction that its exclusive claims are not the product of Western colonialism or ecclesiastical pride but flow from the redemptive plan and activity of God himself. The church must be on mission, because God is on mission.
Flett continues, however, by noting a serious flaw in how missio Dei is often interpreted and worked out in practice:
However, such exuberance is just one side of the story. Commentators describe the concept as, at once, “pivotal” and “confused.” Reference to the doctrine of the Trinity establishes a requisite formal framework, but “God’s mission” fails to draw on this doctrine for its material substance. The resulting vacuity renders missio Dei an elastic concept capable of accommodating an ever-expanding range of meanings. For Wolfgang Gunther, missio Dei functions as a “container term, which is filled differently depending upon each individual author.”… Wilhelm Richebacher illustrates the problem when he observes that missio Dei is used by some to “justify the Christocentric definition of all the mission of the church as distinct from religious propaganda, and by others to do just the opposite, i.e., to propound a deity that bears witness to itself in other religions and thereby counters the absolute claims of Christianity.”…
When compared with the phenomenological underpinnings of missions that were normative at the dawn of the twentieth century, missio Dei is, in truth, pivotal. Without any link to a specific act, however, “mission” soon expanded to encompass the entire horizon of divine and human history. Following Stephen Bevans and Roger Schroeder, missio Dei is “the very mission of God in creation, redemption and continual sanctification.” Every act of God, since God is by nature missionary, is properly described as mission.
Mission, when it did not reduce to a vague involvement within the sociopolitical sphere, very soon became a distilled image of the church’s general direction within history, with the effect, for Hoedemaker, of providing “theological legitimation to the ecumenical emphasis on the church.” Mission was reduced to the being of the church in her mundane operation of word and sacrament, and via an ever-increasing assortment of other practices internal to the church herself. Anything the church did could now be classified as mission. With this, as Stephen Neill famously said, “If everything is mission, nothing is mission.”…
The Copernican turn of missio Dei is not something from which the Christian community can depart. Any other conception of the ground, motive, and goal of mission apart from missio Dei‘s Trinitarian location risks investing authority in historical accident and human capacity. Both the decisive force and fatal flaw of missio Dei rests in its relationship to the doctrine of the Trinity. As propounded to date, the concept is deficiently Trinitarian, and the wide range of its contemporary problems is a direct result of this single lack. Reference to the Trinity distanced mission from every particular human act, but, as now a divine attribute, uncertainty arose over the practical transition from divine being to the human missionary act. Missio Dei‘s vacuity emerges at this precise point….
Missio Dei provides a Trinitarian illusion behind which all manner of non-Trinitarian mediations operate with sanctioned impunity. The Trinitarian formula is pure preamble. This explains plains how a wide variety of seemingly incongruous positions can all lay claim to the name missio Dei.
According to Flett, the reason why missio Dei is incapable of properly grounding Christian mission is due to its inherent lack of a Trinitarian ground and grammar, as T.F. Torrance would put it. God is inherently Trinity — Father, Son, and Holy Spirit. This is who he is by his very nature. When, therefore, we fail to develop our understanding of his redemptive mission with his Trinitarian nature, (i.e. when we separate his act from his being), we will be unable to properly discern how God personally and directly carries out his mission via the church in the world. God redemptively relates to the world as Father, Son, and Spirit, and thus a failure to connect the latter to the former will leave us with a gap which can only be bridged by human activity. If the Triune God is reduced to a simple transcendent monad, then he effectively becomes walled off from creation, and thus his mission effectively becomes dependent in some way on the church. The problem arises, then, that the church becomes sovereign in determining the meaning and methods of its mission rather than subjecting itself to and participating in the mission of God, the very thing that missio Dei was meant to guarantee!
There is a great need, therefore, to develop a fully Trinitarian missio Dei — or perhaps a missio Trinitatis — in order to make missio Dei theology clear and effective. This is simply another way of saying that we need to develop a kataphysic or scientific missiology, one that strictly submits and conforms to the object of its inquiry, namely, the mission of the God who in Christ and by the Spirit has reconciled the world to himself.
 John G. Flett, The Witness of God: The Trinity, Missio Dei, Karl Barth, and the Nature of Christian Community (Grand Rapids, Cambridge: Eerdmans, 2010), Kindle Locations 131-195. For references to sources referenced in the text, see Flett.