At the heart of the ecumenical fellowship stands a moral imperative, a kind of pledge by the churches to hold themselves and one another accountable in their faithfulness. Olav Fykse Tveit argues that this commitment binds each of the churches in fidelity to the larger communion, to the great Tradition, to scripture and the apostolic faith, as well as to the people they serve in the contemporary world. An underappreciated dimension of the perennial quest for unity, mutual accountability functions as a key criterion for the quality of relations within the Christian fellowship. It defines the attitude of active responsibility and openness that churches bring to their fellowship and to their ecclesial self-understanding and renewal. It is a benchmark of their authenticity as churches in the ecumenical age. Tveit’s rich study of the evolving concept of mutual accountability, especially as it figures in the work of Faith and Order over a fifty-year period, traces the growing conviction that ecumenism entails churches’ not simply being together and working together but also holding each other accountable for their genuineness as members of the body of Christ. “Attitudes of openness, constructive critical and self-critical approaches, repentance, reliability, commitment to the common calling and tasks, faithfulness, sharing, and indeed hope—these are all genuine and necessary attitudes in a fellowship that follows the crucified and risen Christ. The truth we owe one another lies in both our insights and our true life in community. . . .” The Truth We Owe Each Other offers a theological underpinning to the work of church renewal and to the continued relevance for the churches’ work in the world.