In brief, global secularisation is not inevitable and, when it does happen, it is not caused by science. Further, when the attempt is made to use science to advance secularism, the results can damage science. The thesis that ‘science causes secularisation’ simply fails the empirical test, and enlisting science as an instrument of secularisation turns out to be poor strategy. The science and secularism pairing is so awkward that it raises the question: why did anyone think otherwise?
What does "why did anyone think otherwise?" mean here?