Religion
Although she doesn't believe in God or organized religion, both play an important role in the life of the unnamed woman at the center of the novel. Growing up, the woman's now-deceased mother (and her extended community, who were very religious) gave her a strong sense of duty and devotion to going to church, believing in God, and promulgating His message. However, after she grew up, moved out of her small-town, and married, those ideals started to fade. She started to become more bitter and less interesting in the machinations of religion. In time, she became agnostic.
That's why, after returning to the town she grew up in, the woman starts to reconsider her past and her past experiences with religion. In other words, she uses religion as a conduit to explore her own past and her relationships with her family and neighbors (but especially with her mother, with whom the reader quickly realizes that she had a unique relationship with). For the woman in her early life, religion was used as a form of repression; as an adult, she used it as a tool.
Grief
At the core of the woman's decision to uproot her life and leave her husband is her grief over the fairly recent death of her mother. Throughout the novel, it becomes clear to readers that the author had a very strange and likely strained relationship with her mother, who died an odd, early death from unknown causes. The grief from her mother's death plagues her; she isn't able to reconcile her difficult childhood and her difficult mother with her current life, but tries her hardest to do just that. In that way, her mother is not just a powerful symbol of her childhood trauma, but her lost innocence and childhood as a result of catering to her mother's every indulgence.
