Anti-Catholicism is part of the dynamics of Northern Ireland's conflict and is critical to the self-defining identity of certain Protestants. However, anti-Catholicism is as much a sociological process as a theological dispute about doctrine. It was given a Scriptural underpinning in the history of Protestant–Catholic relations in Ireland, and wider British–Irish relations, in order to reinforce social divisions between the religious communities and to offer a deterministic belief system to justify them. This article examines the socio-economic and political processes that have led to theology being used in social closure and stratification. It describes the various forms of contemporary anti-Catholicism, and highlights two further sociological features of the process, the common-sense reasoning process which reproduces it and how, in its language, it operates as a ‘discursive formation’.