This is a reprint of Adolf von Harnack's classic book "What is Christianity?" Carl Gustav Adolf von Harnack (7 May 1851 - 10 June 1930) was a German Lutheran theologian and prominent church historian. He produced many religious publications from 1873 to 1912 (in which he is sometimes credited as Adolf Harnack).Harnack traced the influence of Hellenistic philosophy on early Christian writing and called on Christians to question the authenticity of doctrines that arose in the early Christian church. He rejected the historicity of the Gospel of John in favor of the Synoptic Gospels, criticized the Apostles' Creed, and promoted the Social Gospel.In the 19th century, higher criticism flourished in Germany, establishing the historical-critical method as an academic standard for interpreting the Bible and understanding the historical Jesus (see T?bingen school). Harnack's work is part of a reaction to T?bingen, and represents a reappraisal of tradition.Besides his theological activities, Harnack was a distinguished organizer of sciences. He played an important role in the foundation of the Kaiser Wilhelm Gesellschaft and became its first president.