Religion is a big topic, and it's one Chamberlain and his friend Buster Kilrain come back to again and again in The Killer Angels. Chamberlain, a religious man, believes that all people are equal because all have "the divine spark," a soul created by God. This is why he thinks slavery is wrong. Kilrain, however, denies that there's such a thing as a divine spark. According to him, life is just a dog-eat-dog scramble for survival… and he wants to make sure that he gets his.
In the end, after Kilrain dies, Chamberlain reflects on his friend's words: "'No,' Chamberlain said. He was thinking of Kilrain: no divine spark. Animal meat: the Killer Angels" (4.6.18). We're getting two alternatives here: either human beings are created by God and have souls (the "divine spark"), or they're just chunks of meat, fit to get shot without much fuss. After seeing the chaos and carnage of battle, Chamberlain doesn't really seem to have this issue entirely cleared up, but maybe it's not an issue that can ever be solved for sure.
How you define the nature of human beings colors how you think about death, too: Chamberlain, who believes humans have a soul, believes that human souls go to an afterlife in heaven. Kilrain, on the other hand, thinks that death is just the end—you just become nothing. Does this make death scarier? More serious? Is Kilrain especially brave because he fights for his beliefs knowing that he could die and become nothing for them?