Adolf von Harnack was a prominent representative of the so-called Liberal theology, and his 120 years old collection of lectures 'What is Christianity?' used to be very influential at the beginning of the 20th century. In these lectures, Harnack wants to use a purely historical method to uncover the essence of Christianity.
However, his approach seems to neglect not only specific sources, like the Fourth Gospel, but also certain theological concepts, like Christology. After a brief introduction to Harnack himself and his influences, this study critically surveys the collection of lectures focusing on his methodological approach, sources used, and his refusal of Christology.
It also shows how certain decisions Harnack makes during his course through the history of Christianity affected the results he yields. Finally, this study critiques his historical methodology and offers different approaches of contemporary that render Harnack's methodology obsolete.