"The Promise" is a sequel to Potok's best seller, "The Chosen," which also was made into a motion picture. Whereas "The Chosen" takes place during the childhood of the two friends and main characters, Danny and Reuben, "The Promise" follows them from age 18 to manhood. Times are changing and these changing attitudes are found in Judaism as well as in other forms of contemporary life. The boys become challenged by these new interpretations of the Jewish religion.
A major theme of the book is the clash between the old-fashioned strict interpretation of the Jewish religion called Hasidism and the modern Orthodox interpretation of Judaism. The Hasidim adhere to ancient scriptures without deviation. The more modern Orthodox branch of Judaism follows many of the rules, but is more liberal in interpretation of permissible actions and ideas. The two extremes are at odds with each other and this rivalry becomes the basis of the story. It revolves around an adjustment on the part of the characters to modern civilization.
Another theme is the relationship and conflict between fathers and sons. For many years in the Jewish tradition, it was the custom for the son to follow the father's thinking. Danny, for example, grew up in a strict Hasidic family where his father was the spiritual leader of the Hasidic community. It was expected that he follow his father's footsteps and become a Hasidic Rabbi. Danny rebelled and decided instead on a career in Freudian psychology and a more secular life. Reuben's father is less strict than Danny's and is a scholar. Reuben helps researching for his books on the Jewish law, but he nevertheless questions and argues about his father's ideas and interpretation. Even though Reuben becomes a rabbi, he is torn with his interests in philosophy and logic.
A theme to consider is the relationship between a religious Jew and his marriage to a modern secular girlfriend. Danny with his religious Hasidic background falls in love with Rachel who is a modern Jew studying English literature. Danny's family adheres to the old fashion idea where you marry others of similar backgrounds. One of the problems they will face is how to bring up the children.
A theme of the book is employing unconventional methods in psychological treatment. Reuven becomes friendly with Michael, a seriously ill young boy, whose father has been excommunicated from the Orthodox sect because of his critical writing. Michael hates the negative attitudes and criticism placed upon his family and resents his father because of this. He refuses to talk and becomes violently ill. Reuven has his friend Danny, the psychologist, take care of Michael. Danny does so, but resorts to an unconventional treatment of which many other psychologists might not approve. The story to some extent revolves around this approach and the deviation from the norm.
The conflict between Rav Kalman, Reuben's strict teacher, reflects a theme of teacher/student disagreement. Kalman is dictatorial in his belief of the Jewish law and Reuben often challenges him. This almost leads to the denial of Reuben's ordination as a rabbi.