What is Christianity?

What remains of Christianity when stripped of dogma, denomination, and tradition? Adolf von Harnack offers a provocative answer.

In What Is Christianity?, pioneering scholar Adolf von Harnack explores the essence of Christian faith from a historical—not doctrinal—point of view. Based on his influential lectures delivered during the 1899–1900 winter semester at the University of Berlin, this classic work remains a cornerstone of liberal theological thought and a bold challenge to conventional Christian interpretation.

For Harnack, Christianity is not primarily a system of doctrine or ritual, but a living message of eternal life—a spiritual reality experienced in the present, shaped by the life and teachings of Jesus, and valid across all historical eras. Rather than defending a specific creed or denomination, Harnack seeks to uncover what is essential and permanent in the Gospel, beneath the layers of institutional development and dogmatic evolution.

You will explore:

• How the historical Jesus differs from the Christ of dogma

• The distinction between religion of Jesus and later church theology

• Why Harnack saw the Gospel as a message of inner transformation, not outer form

• The tension between historical context and timeless truth

Harnack’s work remains widely studied and debated for its clarity, intellectual rigor, and influence on modern theology. His bold assertion—“Either the Gospel is in all respects identical with its earliest form… or else it contains something of permanent validity”—invites readers to rethink the Christian faith in light of history, not merely tradition.

Whether you're a student of theology, a seeker of authentic faith, or a reader of historical Christianity, What Is Christianity? offers a thought-provoking perspective on what truly lies at the heart of the Christian message.

Adolf von Harnack (1851-1930), was a German theologian and prominent church historian. He produced many religious publications from 1873-1912. 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 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 Tübingen school). Harnack's iconoclastic work is part of this tradition.