This dissertation explores “practical definitions” of religion. A practical definition of religion is how social actors act, refrain from acting, and understand their actions in relation to religion in practice and across various contexts. The project also engages with themes of cognitive sociology, especially how sociologists can best incorporate data on cognition into their studies. It accomplishes these complementary tasks through a multi-sited ethnography of four separate communities: atheists and evangelical Christians in Oklahoma and Chicago. Using a set of mixed methods incorporating participant observation, in-depth interviews, and field experiments, it finds that practical definitions of religion change depending on who interacts with religion and how the interactions take place. ,Chapter 1 outlines the general theoretical framework of the project. Chapter 2 shows how some evangelicals situationally reject the label of “religious” in order to redirect the conversation over what it means to be Christian. However, data from timed field experiments measuring dual-process cognition, in combination with traditional qualitative methods, suggest that despite rejecting “religion” these evangelicals cannot entirely disassociate themselves from the everyday notion of religion as a social category. More generally, the chapter explores the place of dual-process theory within sociology and suggests ways to collect sociologically useful data on dual-process cognition.,Chapter 3 draws on a card-sorting field experiment to examine how people understood the shape of the field of religions. Overall, the chapter suggests that people’s conceptions of religion are deeply intertwined with their understandings of a religion’s politics as well as highly dependent on who they define as potential “allies” and “adversaries.”,Chapter 4 investigates how aspects of group self-concept influence group humor styles. Drawing on participant observation with atheists in Oklahoma and evangelicals in Chicago, it shows that group humor styles are a function of how groups think of themselves, think of key outsiders, and who is present in a given interaction. This process helps explain why some religious minority groups, such as atheists in Oklahoma, have a culture of biting and potentially offensive humor directed at outsiders, while other religious minority groups, like evangelicals in Chicago, use much milder humor that is focused on their own group.