Secularization and secularism are interpretive narratives and analytical systems of locative naming that co-construct the category of religion in spatial relationship to the idea of the secular as not-religion. These approaches were developed in the 19th century to make sense of the social restructuring of industrial societies. They begin with the assumption that religion is spatially identifiable as Christian church space, as readily recognizable in built congregational structures. And they consider the secular, in the most literal sense, as that which is not. That is, the secular is everything physically outside church space. But secularization theorists often do not adhere to this literal interpretation of spatial difference. They also use space metaphorically in their understanding of “disestablishment” as referring to more than just the physical state-expropriation of church land, but also to the separation of spheres that results from nation-state legal sovereignty, particularly focused on the spatial division between secular culture and church subcultures.
Whereas secularization theory offers narrative frames to orient a historical trajectory of religion in relation to not-religion, the study of secularism describes attempts to understand the political and legal regulation of religion in relation to sovereign nation-states. Methodological distinctions between secularization and secularism invoke a long-standing problem in the study of religion: the ability of the scholar to discern the difference between the metaphorical map of religion in relation to the idea of the secular, and the state governance of physical territory.
Classical secularization theory was constructed within the colonial context of the 19th century, and it carries within itself the spatial distinctions that define an Enlightenment conception of the Western nation-state, as a secular sovereignty set apart from and transcendent of the revelatory particularity of religious authority. More recent versions of secularization theory in the United States still assume that only the secular state can transcend physical space and still control its boundaries and borders. Religious transcendence, by contrast, is viewed as otherworldly. The reason for this is because unlike secular authority, which is self-evident and universal, religious authority is revelatory and particular. Within secularization theory, religions then are limited in their ability to physically enact, in every sphere of life, their revelatory mandates. They can do so only as long as they maintain a high level of orthodox belief and practice, to the extent that there is no distinction between religious and cultural authority. Secularization theory thus assumes that religious pluralism of any kind results in a competition to see which religion can control all aspects of life. The nation-state then is viewed as the transcendent mediator of religious claims to civic life and public space. And while secularization theory considers this mediation in the spatial terms of public practice and private belief, studies of secularism give more attention to the historical and contextual limits of nation-state transcendence, as well as the ways in which nation-states physically bound religion as a category, whether as located in the legal limits of 501(c)(3) nonprofit status, or a congregational building with a street address.
Though the term secularism has been a co-generative concept in classical secularization theory, theories of secularism have been more fully developed since the late 20th century. Some of those approaches have extended the spatial concerns of secularization theory, particularly as related to the question of religious endurance as measured in terms of public practice and private belief. The mere difference, which has garnered quite a bit of writing, is to shift the interpretive gaze away from the individual challenge of Protestant Christians to maintain a comprehensive religious meaning-making system, a “sacred canopy,” in the midst of increasing religious diversity, to the ability of “orthodox” religious subcultures to maintain religious authority in the midst of a pervasive secularism that is antagonistic to the possibility of any totalizing religion, one that is lived out in all spheres of life. Other theoretical approaches to secularism, however, are more directly engaged with post-colonial scholarship, and are more focused on the role of the nation-state in the categorical construction of religion, than they are worried about the social loss of traditional religion.