A conversation with Peter Heather (King's College, London) about his new book Christendom: The Triumph of a Religion, AD 300-1300 (New York: Knopf, 2023). Peter is one of the leading historians of the fall of the western Roman empire and the emergence there of the post-Roman, "barbarian" kingdoms. He now brings a revisionist approach to the emergence of the Church in (mostly western) Europe. This book covers a lot of ground, and so we focus on the early period, where his arguments affect the east too. We talk about the role of contingency, the near-miss of Homoian Christianity, the decisive role of secular rulers, and much more.