How would you explain Ruth's zealousness for her new religion?
James McBride's overriding thesis is that all the religions and races are more alike than they are different. Just as water is reflection of all the colors, McBride says that the defining quality of Ruth was her ability to assimilate. She absorbed Judaism and blended it with Christianity. She assimilated her race with her husbands'. This is not to say that she watered down either her religious beliefs or settled for marrying outside of her race; it means that she saw the best qualities in each and amalgamated them into a new set of beliefs and loves.
Ruth was a remarkable woman: she valued education above tradition, and she didn't see race as a limitation. She believed in her choices and commitments to her family. It was her faith in choice that helped her rise above the stereotypes, racism, and guilt in society's religious and marital mores.
Assimilation is the defining aspect of cultures: it is inevitable. Ruth knew that we are all more alike than we are different, and she taught her family to believe it too.