The bloodied God-man—the living exemplar of the "problem of good," as Mendoza puts it—is the most compelling humanism, among all humanist attempts to resist the encroachments of all manner of post- and non-humanisms. In a film with the persistent suggestion that both evil and sacrificing for the good of others drains us, the sacrifice gains a strange, hopeful significance when underscored by a suffering God-man. One of these self-emptying actions may not, in the end, destroy us.
So to understand Derrickson's foray into the devil's territory is to understand that he doesn't want us to revel in darkness; he wants us to face our fears. And if we avail ourselves to a powerful enough influence, we just might dispossess ourselves of our very worst anxieties.
Deliver Us from Evil is rated R for persistent horror, grotesque images including dead cats, moderate violence including gun fire and knife fights, disturbing flashes of evil including a dead baby and domestic violence, and quite a few instances of cussing including a few uses of the f-word. All of it is frightening and appropriate to the film's narrative/setting. For mature adults.