The main character in Chaim Potok's The Promise, Reuven Mater, greatly resembles another of Potok's fictional creations, Asher Lev, in some ways, but couldn't be further away from him in others. The problems they face and the way they each handle them, coupled with their strong Jewish upbringings greatly link the two books and help to understand them on a higher level.
Reuven, like Asher, throughout the start of the novel, was forced into dealing with many conflicts that came about because of his strong connection with his religious faith. For instance, he was looked down upon for dating the daughter of a man who had written numerous controversial religious books. Also, by using controversial Talmud-interpreting methods learned from his father, Reuven was under constant criticism by his professors and his community. Although these types of problems differ substantially from Asher's due to the great difference in their ages, because of the connection the problems have with religion, each of their communities sees the handling of these problems as a reflection of their Jewish faith.
And because of their differing levels of maturity (something the community does not seem to consider in judging them), the two characters, as expected, handle their problems very differently: Reuven does it in a collective manner by justifying himself in every way possible while Asher tends to feel sorry for himself thus digging himself deeper into holes (the ones created by his parents). However, in spite of this, both Reuven and Asher can never seem to resolve their problems (no matter the effort they put in to doing so) because new problems always seem to surface. Obviously, this frustrates them greatly because they are problems fabricated by the people around them who are unwilling to change, and not solely by their own actions.
While the similar situation the two are in connects them, their understanding of their roles in the Jewish community separates them far more. Reuven has developed a keen comprehension of what it means to be a practicing Jew which has earned him enough respect from others to help him conquer his problems. However, Asher has yet to find that understanding and is suffering greatly because of it. And unless he begins to realize his error, his situation will only get worse.