In this novel, participation within one's community becomes more than an enjoyment—it is now an absolute necessity for survival. The Unconsecrated, whose name begs a religious question, stand in for primordial chaos, and the goal of the community is therefore to establish borders (the fences) and to monitor them for breaches. When chaos interrupts, people die and the whole community is threatened. Mary stands on the edge of that chaos by accepting a role in the Cathedral (another religious motif) where she encounters the chaos of community one person at a time.
To say that the Unconsecrated were the only arbiters of chaos would be misreading the symbolism. The community is chaotic and often very destructive itself. For instance, Jed bans Mary from their house after Mary's mother has just died. He sends her away when she needs his support most, rejecting her in a mistaken passion. That passion is within each character, so in addition to building an external wall where the chaos from the zombies is held at bay, they must also erect fences internally, protecting their selves from each other, and then, in moments of intimacy, they reveal themselves.
The most clear instance of this is probably when Mary meets Travis. Travis is an old friend from youth, and he is wounded, so Mary feels a kinship with him, because in a way, she is wounded too. Plus, their childhood friendship is a symbol of the importance of this conversation. What does she tell him? She tells him her mother's old stories about the ocean—a threefold symbol of chaos. She shares stories about her mother (the foreign feminine element to Travis who probably understands fathers better than mothers), her stories about the ocean (the ocean being a classic symbol for chaos), and they are now the stories of a dead person, because the mother has died, making death the ultimate sign of chaos. Indeed, a breach from the Unconsecrated does mean death, so the symbol is precise.
