This book explores why progressive Christian mission in the 20th century increasingly focused theologically, and structurally, on themes of community. It focuses on two case studies, the Iona Community (1938) and the Council for World Mission (1977) which both adopted new structural models to meet their distinctive missional contexts. The work sits in the fields of World Christianity and Missiology and uses archival research to outline the evolution of both the case studies in the 20th century. The comparison of the two UK-based mission agencies, both in the Reformed theological tradition, one predominantly functioning in the UK, and the other active outside of the UK, has revealed that both developed paradigms of mutuality to deliver mission on a bi-directional pattern—aiming to meet the needs of a missional movement struggling with global decolonization and its relevance to a British society that was shocked by suggest two world wars and the economic turmoil of the Great Depression.