Introduction to Christianity.

MLA

Ratzinger, Joseph, and J. R Foster. Introduction to Christianity. New York: Herder and Herder, 1970.

APA

Ratzinger, J., & Foster, J. R. (1970). Introduction to Christianity. New York: Herder and Herder.

Chicago

Ratzinger, Joseph, Introduction to Christianity.. Translated by J. R Foster. New York: Herder and Herder, 1970.