Symbolism, Imagery, Allegory
In addition to having a whole bunch of religious significance, the Virgin Mary is the novel's ultimate mama figure, seen as a source of strength, guidance, and comfort.
One representation of the Virgin Mary, the statue known as Our Lady of Chains, is a particularly powerful symbol of those elements. When Lily first sees the statue, she is awed by the strength and history it conveys:
She was black as she could be, twisted like driftwood from being out in the weather, her face a map of all the storms and journeys she'd been through. Her right arm was raised, as if she was pointing the way, except her fingers were closed in a fist. It gave her a serious look, like she could straighten you out if necessary. (4.29-30)
Legend has it that statue (which was the old figurehead of a ship) washed up near a South Carolina plantation during the era of slavery, and the slaves who found her decided she was a representation of the Virgin Mary. From that point on, the slaves drew hope, inspiration, and strength from her, and she was even credited with helping several escape.
As she feels the loss of her own mother acutely, Lily is attracted to Our Lady's potential to serve as a mother figure her and others. Visiting the statue late at night, Lily thinks:
I live in a hive of darkness, and you are my mother, I told her. You are the mother of thousands. (8.234)
August, too, thinks that Our Lady could serve as a stand-in mother for Lily, slipping the suggestion subtly (very subtly—so much so that Lily totally misunderstands the moral) into her story about Beatrix the nun, for whom the Virgin Mary stood in when she ran away from home.
In short, the novel's representation of the Virgin Mary is far from traditional, converting her from a purely Christian figure into a kind of earth mother goddess who possesses both historical significance and the power to serve as a "mother" to all who reach out for her.