In this study, Teun van der Leer tells the story of the Believers' Church Tradition, a tradition, mainly rooted in the so-called Radical Reformation, which prefers to be called a movement, or rather a renewal movement. Its name is a program, a vision, and a way of being church. Based on extensive source research, this book describes and analyzes the defining characteristics of this so-called "third type of church" and investigates its ecumenical value. With an extensive description of its nature of faith, the church, hermeneutical discernment, and mission, this book colors a movement within the church landscape that has never been mapped in such detail before. As such, the book provides an in-depth introduction to this ecumenically important but still a bit underexposed movement and makes a substantial contribution to the ecumenical ecclesiological debate about the church and its future.