Harnack's Essence of Christianity, and What It Set Up

In the winter of 1899 to 1900 Adolf von Harnack, the leading church historian of his generation, delivered a series of unscripted public lectures at the University of Berlin attempting to say, for a general audience rather than a specialist one, what the essence of Christianity actually was once every layer of later doctrinal development was set aside. Published as What Is Christianity?, the lectures reduced that essence to three linked claims: the fatherhood of God, the infinite value of the individual human soul, and the higher righteousness of the kingdom of God, understood chiefly as love of neighbour. Councils, creeds, and later Christological formulas Harnack treated as the historically conditioned wrapping Greek philosophical categories had placed around a simpler Galilean message, useful to their own moment but not part of the essence itself.

The lectures were read across Europe and became the most visible public statement of what liberal Protestant theology actually claimed, which is exactly why they became a target rather than only an achievement. Karl Barth, who had studied under Harnack, came to regard the lectures' confidence that a scholar's own historical judgement could recover Christianity's timeless essence as the clearest instance of liberal theology putting human reason in the seat that belonged to revelation alone, and Barth's break from his teacher's whole approach after 1919 is unintelligible without this lecture series as its immediate backdrop.