This is part of a series based on chapters four and five of my new book, The Church Is Flat: The Relational Ecclesiology of the Emerging Church Movement, in which I look at the ecclesiology of German theologian Jürgen Moltmann and put it into conversation with the ecclesial practices of the emerging church movement (ECM). Part One
Key to understanding Moltmann’s ecclesiology is to grasp his concept of the social trinity. The ecclesiology that he proposes, and that I take up in my book, is a “relational ecclesiology,” and he gets there by beginning with a concept of the godhead that is fundamentally relational.
For Moltmann, God is relationship — primarily the relationship between Father, Son, and Spirit.
Significantly for Moltmann’s ecclesiology in The Church in the Power of the Spirit, and growing out of his reflections on the crucifixion (in The Crucified God) and resurrection (in Theology of Hope), he develops a social understanding of the Trinity, a theme that overarches all six of his later “contributions” to systematic theology.
Moltmann rejects traditional Western trinitarian theories—those held both by the Catholic Church and most Protestants—as “modalism” or “monarchial monotheism” and instead adopts the Eastern conception of God as a perichoresis, or an enduring and mutually interpenetrating fellowship of divine love between three persons. For instance, the Holy Spirit is not merely the power between the Father and the Son, but is instead a unique subject (Moltmann strongly rejects the eleventh century creedal addition of the “filioque clause”).
One characteristic of Moltmann’s perichoretic Trinity is that of openness: the fellowship of Father, Son, and Holy Spirit is “other than” creation, but it is open to embrace creation into itself. Thus, Moltmann argues that all of history is actually a part of the “trinitarian history of God.”
So, understanding Moltmann’s particular view of the Trinity is essential to understanding his ecclesiology.
 This phrase, which becomes very important in Moltmann’s writings, is first introduced in Moltmann, The Crucified God: The Cross of Christ as the Foundation and Criticism of Christian Theology, 274ff.