Sociologists now treat the classical secularisation thesis as having failed in an important respect, while insisting that religious decline in western Europe is real. How can both of those be true?

C. Decline is real in western Europe; the error lies in generalising it to modernity everywhere.

The measurements held; the inference did not. Attendance and membership fell steeply in western Europe, while the United States, Latin America, sub-Saharan Africa and much of Asia went the other way. What failed was the assumption that modernisation carries religious decline with it as a matter of course.