What separates faith from heresy? Who has the authority to define truth in the formative centuries of Christianity? Essential Heresies: The Struggle of Early Christianity offers a critical examination of the major doctrinal controversies that defined the first centuries of the Church. From disputes over the nature of Christ to the condemnation of Gnostic, Marcionite, or Arian currents, this work examines the theological, political, and ecclesiological mechanisms that set the boundaries of orthodoxy. Drawing on patristic and conciliar sources as well as contemporary scholarship, Agustin V. Startari argues that the controversies later classified as heretical were not marginal anomalies, but decisive factors in the clarification of dogma, the delimitation of communion, and the consolidation of forms of ecclesial authority. A rigorous and essential work for understanding how the Church was born in tension, amid doctrinal conflict, the search for unity, and the struggle for truth. Agustin V. Startari is a Uruguayan researcher and author. His work focuses on the history of ancient Christianity, medieval political theology, and structures of religious power. He has published books on the Templars, the papacy of Gregory X, and philosophical cosmology, among other subjects.