In this definitive study, Adolf Harnack explores the life and influence of Marcion, a figure who challenged the very foundations of early Christian thought. Known for his rejection of the Old Testament and his provocative views on the nature of God, Marcion remains one of history's most misunderstood and controversial figures. This scholarly work delves into his theology, the formation of his own canon, and the fierce opposition he faced from the early Church. By examining primary sources and historical context, the authors provide a clear window into the intellectual battles that shaped the development of the New Testament. Whether you are a theology student, a researcher, or a history enthusiast, this book offers essential insights into how early Christian doctrine was defined in response to heresy. With helpful bibliographies and indices, it is a crucial resource for anyone seeking to understand the formative years of the faith and the critical debates that ensured the preservation of orthodox doctrine. Explore the theological landscape of the second century and see how these ancient challenges continue to echo in modern discourse.