Joe Christmas is the closest to a protagonist that we have in Light in August. He is a young man growing up biracial in the American South. He's abandoned by his grandfather and sent to a white orphanage where he gets taunted by the other white children there because he is darker than they are. Later, Miss Atkins repeatedly calls him a "nigger bastard," and sends him to live with a strict, abusive Calvinist named McEachern, who teaches him to accept violence as a normal mode of interacting with other people.
It's important that Christmas's racial identity remains ambiguous. Because there was no way to prove ancestry at this time in American history, so much of racial identity was determined by what white society chose to think about individuals who looked darker. Christmas can never confirm nor deny anything about his history, so all he has to go on are the words of racial fanatics like Mr. Hines and Miss Atkins. Christmas wanders through the novel playing the part of both a black man and a white man, whenever it suits him best. But while this might seem like an advantage in today's cosmopolitan culture, it actually played out more as a curse in the 1920s, since Christmas was doomed to feel uncomfortable no matter where he was.
Christmas is repeatedly described in Christ-like terms. For one thing, his initials are J.C., which calls to mind Jesus Christ, and he dies at age 33, just like Jesus did. In addition, throughout Chapter 7, as McEachern beats him, Christmas is described as "a monk in a picture" (7.9), "a Catholic choir boy" (7.10), a hermit (7.20), and while he's beaten he is described as "rapt" and in "exaltation" (7.9; 7.10). It almost seems as though he enjoys suffering and is being written as a martyr-figure. When he dies, the novel insists that everyone in Jefferson will remember Christmas forever, and his death is given a note of transcendence.
It's just not clear what Faulkner intends with all of the Christian imagery, however. Christmas is a mean, misogynistic, and violent character who doesn't try to redeem himself, nor does he try to redeem anyone else. He is certainly not a spiritual figure, and he actually spends much of the novel trying to avoid Christianity. These facts make it difficult to see him as a purely Christ-like character, and the reader is left to make sense of all of these contradictions.
Christmas seems to lose his grip on reality once he realizes that he'll always be uncomfortable in his own skin, and that he will always be uneasy in the society in which he lives. Miss Burden's attempts to "reform" Christmas by turning him into a devout black activist only alienate and confuse him further. While he is uncomfortable in white society, he seems unwilling to give up his position "passing" as a white man, and the idea of helping black people seems disgusting to him. This inability to reconcile his simultaneous desire for and hatred of both whiteness and blackness ultimately lead him to kill Joanna, and to allow his own death to take place.