https://en.wikipedia.org/wiki/Taiping_Rebellion?wprov=sfla1
The rebellion was deadlier than WW1 by some estimates. Was it just a case of dissatisfaction leading to following whoever was opposing the government?
The way you've phrased the question already, I think, suggests that you're not unaware that religions don't simply attract followers for existing. This aspect is highlighted in a previous answer of mine, but if you have further follow-ups, do please ask them, as I'd be more than happy to go into as much depth as you'd like (well, up to the limitations of the source material anyway.)