In What is Christianity?, Harnack defined the essence of the Christian faith as the universal fatherhood of God and the universal brotherhood of man. Harnack’s indebtedness to liberalism, humanism, and universalism is apparent in his book as it denies the exclusivity of Christ. To be true to Scripture, we do affirm the universal Creatorhood of God, for all people are made by Him (Acts 17:28). However, He is Father only of those who believe in His only-begotten Son — the Lord Jesus Christ. This is the point of John 1:12–13.

“But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.” (John 1:12–13)

Near the end of the nineteenth century, the discipline of comparative religions was incorporated into universities across the Western world. The advances in technology and travel dramatically “shrunk the world,” thereby bringing previously isolated beliefs and practices into interaction. Therefore, people began to study and compare the different world religions arriving in their neighborhoods.

During this time, scholars attempted to find the essence of all belief systems. German liberalism was particularly keen on identifying the wesen, or substance, held by all world religions. Many of these thinkers said that all religions shared the same end, and so professors began seeking for this common denominator. Adolf von Harnack, a famous professor of church history, wrote a book that perfectly models the thought of this period.

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