I know that the Taiping Rebellion was started by a guy who thought of himself as the brother of Jesus Christ. But how did such an idea arise inside an insular society like 19th Century Confucius Qing Dynasty China? How did he gain so many supporters as to capture much of Southern China and lead to the deadliest Rebellion in history? It all seems like something out of a novel or a fact that sounds too weird to be true. How did it really happen?
Your sub-question about how Hong gathered his followers is one I've answered recently, but I'd be happy to add more or answer followups.
I do, however, want to push back a bit on the idea that 19th century China was insular, or indeed Confucian. The Qing Empire, which was established in 1636 and conquered most of the Ming Empire by 1662, was founded by Manchus who operated in a distinct set of religious traditions. Confucianism remained the norm in China, but the wider Qing empire was run on relatively particularistic terms. The Qing leadership was also very actively involved in world affairs, and had a particularly active frontier policy. This is also something I mention in the podcast episode (part 1) (part 2) I did with /u/steelcan909 about a year ago.