For most of the twentieth century the field's organising expectation was secularisation: that modernity, meaning industrial economies, urbanisation, mass education and science, would steadily reduce the social importance of religion, and that the pattern visible in Western Europe was the future everywhere.
It did not happen that way, and the retraction was unusually public. One of the thesis's most prominent exponents stated in the 1990s that he and most of his colleagues had been wrong, and that the assumption of a link between modernisation and decline could not be sustained against the evidence. The United States remained highly religious while becoming highly modern. Pentecostal and charismatic Christianity grew rapidly across the global South. Political Islam became a major force. Religion in Eastern Europe survived decades of state atheism and returned.
What replaced the thesis is not a single successor. One line of work treats religious institutions as operating in something like a market, with pluralism and competition producing vitality and state-supported monopolies producing complacency, which accounts for the American case reasonably well and other cases less so. Another argues that what declines with development is not belief but the authority of religious institutions over other spheres, which is a narrower and more defensible claim.
Western Europe now looks like the case requiring explanation rather than the template, which is a considerable reversal for a discipline founded largely in it.