This article offers a critical reflection on the World Council of Churches Conference on World Mission and Evangelization in 2018 in Arusha, asking for a renewed focus on and discussion of ecumenical and missional theology, especially the relation between unity and diversity among churches and Christians. Based on an outline of the ecumenical discussion on “unity and (reconciled) diversity,” the author asks for a strengthened focus on the unity of the church. She claims that transforming unity is important for the credibility of the church in its communication of the gospel to the world, that transforming unity is a “matter of survival” in times of secularization and in times of religious persecution, and that transforming unity implies a call to diakonia as an unnegotiable part of God’s mission to the world. Concluding, the author asks for further theological discourse on important themes lifted up by the Arusha Conference, like “mission from the margins,” the work of the Holy Spirit (pneumatology), deification/sanctification/theosis, and the relation between unity and diversity in worship and spirituality. Finally, the author underlines that the Arusha Conference was an important reminder of God’s call to the churches for the ongoing mission of transforming unity.

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