The fundamental structure and elements of the thought of Adolf von Harnack and his relation to Christianity are best represented in the controversial collection of his lectures from the years 1899-1900, which was issued under the title Das Wesen des Christentums (1900) (English translation, What is Christianity? 1901) and has been considered as one of the principal writings of liberal Protestantism. The present study concentrates on the overview of Harnack's essential reflections on Christianity, including their most serious critics.