How can theologians recognize the church as a historical and human community, while still holding that it has been established by Christ and is a work of the Spirit? How can a theological account of the church draw insights and concepts from the social sciences, without Christian commitments and claims about the church being undermined or displaced? In 1927, the 21-year-old Dietrich Bonhoeffer defended his licentiate dissertation, Sanctorum Communio: A Theological Study of the Sociology of the Church. This remains his most neglected and misunderstood work. Christ Existing as Community thus retrieves and analyses Bonhoeffer’s engagement with social theory and attempt at ecclesiology. Against standard readings and criticisms of this work, Mawson demonstrates that it contains a rich and nuanced approach to the church, one which displays many of Bonhoeffer’s key influences—especially Luther, Hegel, Troeltsch, and Barth—while being distinctive in its own right. In particular, Mawson argues that Sanctorum Communio’s theology is built around a complex dialectic of creation, sin, and reconciliation. On this basis, he contends that Bonhoeffer’s dissertation has ongoing significance for work in theology and Christian ethics.