The lectures on the essence of Christianity that Adolf von Harnack held in Berlin in 1900 are to this day classic reading in the field of religious science. Their new translation, with a valuable introduction and accompanying documents, provides an opportunity to come back to this famous text and study its reception. By insisting on an absolutely historical reading of the Christian religion, Harnack became the founder of a postdogmatic approach, suited to the intellectual demands of a new century. But Troeltsch, through a critical approach to this “pure” historicity, reveals the ambiguities of the notion of essence, which looks backwards to the time of origins but is at the same time concerned with implications for the present. Thus opens this capital file on the relationship between theology and history on the threshold of modernity.