Harnack's lectures on the ‘essence’ of Christianity were delivered at the University of Berlin during the winter of 1899–1900 and published shortly afterwards. Their success was immediate and Das Wesen des Christenthums was repeatedly reprinted and translated. The widespread controversy it aroused was but further testimony to its importance as an expression of current liberal Protestantism. The deep sincerity of its author, one of the most eminent scholars of his generation, combined with the simplicity and directness of its approach, marked it out from the first as a work of historic significance. This reputation it has maintained, even if it is seldom read today. (The English translation, What is Christianity?, by T. B. Saunders has, however, been reprinted in recent years in two different editions.) As a document of late nineteenth-century religious thought it is of the highest interest.

Adolf von Harnac—he was ennobled in 1914 by the Kaiser Wilhelm II— was born at Dorpat in Estonia in 1851, the son of a professor of pastoral theology. He studied at the university of Leipzig, later joining its teaching staff. He himself occupied professorial chairs, first at Giessen (1879) and Marburg (1886) and then, with international fame, at Berlin (1889–1924). From 1905 until 1921 he was Director of the Prussian National Library and for ten years (1902–12) President of the Evangelical Church Congress. He died in 1930.

His opus magnum is his monumental Lehrbuch der Dogmengeschichte (1886–90; 6th ed., 1922; reprinted 1963), an English translation of which was completed in 1899.