How did Hong Xiuquan go from a guy failing his exams to leading a theocratic absolute monarchy ruling millions in just a few years?

Your question is a very good one, not helped by the relative paucity of internal source material discussing this early period in the Taiping's history. It is worth noting that the Taiping's own documentation is not always helpful for this early period, as contemporaneous writings are rare and so much of the detailed material is later (and thus open to some suspicion as to their contents). To my knowledge, there has not been much attempt to make use of either Qing official correspondence or of local histories, at least in the English historiography. As a result, explanations tend to be somewhat general and even to an extent speculative.

I think the first thing to point out, though, is that the size of the God-Worshipping Society by 1851 was not particularly vast – the most generous estimates put them around 50,000, and the more realistic upper end might be around 30,000. If we were to run a bit of maths on this, you'd only need an annual growth rate of around 260% – that is, 2.6 new converts at the end of the year for each existing member at the start of it – to go from 1 person to 30,000 in 8 years, assuming a steady exponential rate of growth. Considering that Hong Xiuquan converted at least 3 people in 1843 (and many more if we also count his more immediate family members as 'converts'), he was pretty well on track!

Still, how did the Taiping manage to achieve such a growth rate? For this we need to understand something about how secret societies like the God-Worshippers, and their longer-established competitors, the Heaven and Earth Society (or Triads), developed. Secret societies broadly had three main aspects to them, and different societies prioritised them to different extents at different times. Firstly, they had a religious, or at least ritualistic component, serving spiritual needs that official public ritual may have failed to. Secondly, they functioned as mutual aid organisations, pooling together members' resources for times of necessity and for protection against official corruption and even armed banditry. Thirdly, they often developed an anti-Qing agenda, perhaps through seeing their success at managing local communities where the Qing had failed as a sign of legitimation. While these three elements did not have to emerge sequentially, it was almost invariably the case that secret societies began as religious mutual aid groups, and only began to explore political dissent after being reasonably established. In other words, secret societies refocussed the uncertainties and discontentment of an existing membership into dissent, rather than initially capitalising on an existing dissenting mindset. This was true of the Heaven and Earth Society, and it would also be the case for the God-Worshippers.

So to begin with, that leaves us with two questions: Firstly, how did the God-Worshipping Society relate to existing local religious currents in southern China? Secondly, what sort of aid did the God-Worshippers provide, and why was it needed?

The Taiping had a complex relationship with local religion in south China. Taiping ritual was very much rooted in conventional Chinese religious practices: among the more visible the offering of edible 'sacrifices' during services, and burning prayers after they were written on paper. But their reuse of ritual conventions did not entail a full acceptance of established popular religion. Instead, while the Taiping accepted many of the conceits of south Chinese folk religion, they also challenged much of how it was conducted, either creating or exacerbating the perception that existing religion was proving inadequate to the needs of local societies, and offering their own religion as a superior alternative. This is borne through in a few examples: for instance, over the winter and spring of 1847/8 there was a spate of God-Worshippers who claimed to be able to channel various heavenly spirits. Eventually, Hong Xiuquan and his cousin Feng Yunshan chose only to recognise two of these mediums: Yang Xiuqing, channelling God, and Xiao Chaogui, channelling Jesus. On the one hand, local religious practice was integrated into that of the God-Worshippers, but on the other, it took place specifically on Hong Xiuquan's terms, and only included the primary deity and Jesus at the expense of any lesser spirits. The other major case that springs to mind is the exorcism of a deity known as King Gan, whose image was housed in a cave near Xiangzhou, on 16 October 1847. Even in the official Taiping account from 1862, King Gan (referred to as Gan the demon) is acknowledged as a real evil spirit with real powers, but which was overcome after Hong Xiuquan beat the idol and pronounced its occupant guilty of various crimes, then had the idol mutilated, and finally wrote several verses condemning the spirit on the cave walls. To put it a certain way, these local beliefs were true but incorrect: there were demons, but they ought to be destroyed, not appeased. In this way, the Taiping managed to both integrate and challenge local religion simultaneously. Joining the God-Worshippers did not mean abandoning one's entire religious outlook, but rather taking a new approach within the same framework.

As for mutual aid, the Taiping entered a situation in great need of it. There was a general competition over limited land and resources – well, in a largely agrarian economy these were one and the same – owing to a boom in population under the Qing. This drove a number of people off the land and into banditry to survive, while many of those who did still work the land worked increasingly small plots, and were increasingly reliant on plots rented from major landowners instead of land they owned themselves. There was also some impact – though the extent is open to debate – from a decline in riverine commerce owing to the reduced importance of Canton as a trade port following the opening of Shanghai, and also the damage caused to the Qing army in Guangxi by the war with Britain. Inequality also exacerbated identity divides, as the Hakka people, a numerical minority in Guangxi as a whole, tended to get short shrift compared to the majority Punti (or Cantonese). The Hakka-Cantonese divide widened until, by the early 1850s, it reached a point where there was open raiding between Hakka and Punti communities. Our exact chronology for how the Taiping responded to the ethnic side of things is a little shaky: it is normally speculated that they originally emerged as specifically defending the Hakkas, but that by 1851, they had broadened into a general militia movement protecting Han people of all stripes, potentially linked one way or another to the emergence of their anti-Qing agenda. Aside from providing military defence, the Taiping did also engage in some financial aid for members, although the main known case was the 'Charcoal collection' of 1848, where the contributions went towards paying bail for the recently-arrested Feng Yunshan. So the Taiping clearly were a functional mutual aid group in the mould of other secret societies, and apparently a particularly successful one.

I've alluded to the transition to anti-Qing rhetoric a few times now, so I probably ought to discuss that. The best analysis of specifically Taiping identity politics is that of Huan Jin in her 2018 article 'Violence and the Evolving Face of Yao in Taiping Propaganda', in which she notes that the use of yao ('demon') to describe Qing supporters only first really began to take hold in any public-facing Taiping writings around 1853, when the first compilations of anti-Qing placards were published. These placards identify both Manchus and their supporters as yao, and only fully narrowed to mean the Manchus specifically by around 1860. But beforehand, yao could realistically mean any threat to the movement. All this to stress, really, that there was likely a gradual transition from a potentially Hakka-centric mutual aid group to a more pan-Han, anti-Qing (and by extension anti-Manchu) movement over the course of the God-Worshipping Society's existence.

One final point to make is that Hong was not the absolute leader of the Taiping any time before 1856, because the origins of the Taiping, particularly in how they absorbed local religious practices, entailed quite a diverse set of loci of spiritual authority. Reading against the later narratives a bit, it seems that Hong, who only started to helm the God-Worshipping Society around 1847, was very reliant on Feng Yunshan, who was the society's actual founder in 1844 – after all, it was both Hong and Feng who assessed the validity of the spirit channellers' claims in 1848. Moreover, the two spirit channellers who passed the test, Xiao Chaogui and Yang Xiuqing, were incredibly powerful – after all, Jesus outranked Hong, and God was, well, God! While both Feng and Xiao died in action between 1852 and 1853, Yang Xiuqing seems to have lorded over Hong using his God persona, leading to his eventual assassination in 1856. But until that point, Hong had ruled as the most senior of a group of spiritual and political leaders, rather than being sole charismatic ruler.