In identifying the aspects of Judaism that he considered compelling for modern American Jews, Mordecai Kaplan focused on the human part of Judaism: community and folkways, rather than commandments and spirituality. In contrast, JTS professor Abraham Joshua Heschel sought to reawaken American Jewry to the power of mystery and the imperative to respond to God’s presence in the world.

Further reading:
Mordecai M. Kaplan, Judaism as a Civilization, (3-15, 173-208, 431-435);
The Meaning of God in Modern Jewish Religion, Chapter 1
Abraham Joshua Heschel, God in Search of Man, 3-4, 167, 184-199