Mordecai Kaplan's journals from 1913 to 1934 offer a window into the mind of a tormented and lonely Jewish thinker. As a pioneering theologian, sociologist, and teacher of American Judaism in the twentieth century, Kaplan (1881–1983) stood as a towering figure at the Jewish Theological Seminary in New York City, where he worked for a good deal of his very long life. Yet even with the publication of his groundbreaking work Judaism as a Civilization (1934) and his popular following, he felt marginalized and embattled throughout his life. To help manage and defend those professional conflicts, Kaplan turned to his journal to record his personal struggles and anxieties. These diary entries offer important clues to the ways he discovered and created an American Jewish identity through the art of journal writing.