This is a book about exile. Though this concept is subtlely rendered, exile is central to the meaning of the novel. You might research Biblical stories of exile and explore the resonance between those episodes and the novel.
Another topic explored in the book is the idea of how religion functions within and, in part, defines communities in America in the mid-20th century. There are many scholarly works on this subject, I believe, and the results of might be very instructive regarding Robinson's novel.