APA (7th ed.) Citation

Harnack, A. (1957). What is Christianity? Harper & Brothers.

Chicago Style (17th ed.) Citation

Harnack, Adolf. What Is Christianity? New York: Harper & Brothers, 1957.

MLA (9th ed.) Citation

Harnack, Adolf. What Is Christianity? Harper & Brothers, 1957.

Warning: These citations may not always be 100% accurate.