Of all the Jewish thinkers that arose in the 20th century, none stands out as revolutionary and as controversial as Mordecai Kaplan. Kaplan was paradoxical. He maintained high levels of personal halakhic observance and, in addition to his studies at the Jewish Theological Seminary, also obtained traditional ordination.

He served as an Orthodox rabbi for a time and was part of the founding of the Orthodox Young Israel movement. Despite his commitment to Jewish life as generally recognizable to traditionally oriented Jews, his theological predilections were such that the fundamental nature of Jewish religious identity was transformed in his reconstruction of Judaism. His works Judaism as a Civilization and The Meaning of God in Modern Jewish Religion and the philosophy behind them highlight very different notions of God and Torah and Israel.

While Kaplan was vilified by many Orthodox and finally excommunicated by the rabbis of Agudath Israel, not all his right-of-center critics were ready to dismiss completely, if not the motivation behind, Kaplan's restructuring or reconstruction of Judaism, or the very issues he addressed. Kaplan’s criticisms of contemporary expressions of Judaism were equally distributed. His impact, for good or ill, remains to this day.