Decline of religion

[Mr Greene]

Good education, and expanding knowledge will always chip away at superstition, but if people are happy and content and have what they need, then this goes a long way to negating the need for those superstitions in the first place. There's a reason that religions are in decline in developed democracies, but increasing in the poorest regions of undeveloped countries.

Poverty, ignorance, and lack of opportunity, are all catnip to superstition.

Given the statistical decline of Christianity in Britain the collapse projects back to a point in the late 1910s or early 1920s.
Looking at the period in more detail the principle thing in religion at the time was something called 'Revivalism'.
This looks likely to be the prime suspect for 'the thing that killed christianity'.