This article on the mission theology of the church, a personal perspective by the vice-moderator of CWME, draws on documentation produced by the commission and also responds to the Faith and Order document, The Nature and Mission of the Church. It is based on the trinitarian paradigm of mission referred to as missio Dei, which emphasizes the priority of God's sending activity in the world, by the Son and the Spirit, and the contingency of the church and its mission activities upon that. Therefore, it is concerned with the participation of the church in God's mission to and in the world, and from this perspective, has a particular interest with the actual, empirical church rather than the ideal church, recognizing that the church exists in many different forms in particular social, cultural, economic and political contexts. The article argues that the church is “missionary by its very nature”. Both theologically and empirically, it is impossible to separate the church from mission. Indeed mission is the very life of the church and the church is missionary by its very nature the Spirit of Christ breathed into the disciples at the same time as he sent them into the world. The mission theology of the church as it has developed in ecumenical discussion over the 20th and early 21st centuries is discussed in terms of the relationship of the church to the three persons of the Trinity: as foretaste of the kingdom of God; as the body of Christ; and as a movement of the Spirit. The article shows that being in mission is to cross the usual boundaries and bring new perspectives from outside to bear, and this is a never-ending, enriching process.