For most people, joining the Taiping did not require conversion to a conventional form of Christianity as an intermediate step. Indeed, the Taiping did not see themselves or present themselves as Christians, and the Taiping religion was in many ways divorced from the evangelical Protestantism that inspired it. In a sense, Hong did not 'rally... people under Christianity', he rallied them under a comprehensive programme of social and political reform in which the religious foundation was partly Biblical. With that framework in mind, we can begin to see how people might have joined the Taiping despite the relative non-prevalence of Christianity.
The Chinese term for Christianity is Jidujiao 基督教 (lit. 'Christ Teaching'), and by 1850 all three of its major branches were present in some respect or another – Tianzhujiao 天主教 (lit. 'Heavenly Lord Teaching'), a.k.a. Catholicism; Dongzhengjiao 東正教 (lit. 'Eastern Orthodox Teaching'), which should be pretty obvious; and Xinjiao 新教 (lit. 'New Teaching'), or Protestantism. The God-Worshipping Society (Bai shangdi hui 拜上帝會), which was the original religious organisation in the 1840s that in turn became the Taiping Heavenly Kingdom, did not explicitly associate with any of them. Moreover, the Taiping seem not to have recognised Christians as following the same religion, or at least not initially. French reports allege that the 200-odd Catholic converts of Nanjing, who sheltered in their church when the Taiping stormed the city in 1853, were subject to especial abuse for a time, and when a British diplomatic mission arrived at the city aboard HMS Hermes later that year, according to the official reports the Taiping seem to have been genuinely pleasantly surprised that the British 'worshipped the same god' – not, notably, 'followed the same religion', implying that the question was about bai 拜 ('worship') rather than jiao 教 ('teaching').
The linking of the Taiping to Christianity was thus a largely external process, and one that began quite early on. A newspaper report from the Overland Friend of China regarding rebel activity in May 1851 remarked upon the rebels being led by a Catholic convert, and dismissed rumours that he was a Protestant. Irrespective of the specific designation, this was an imposed rather than a self-professed one, and from there on out Western missionaries, mercenaries, civil servants and politicians would debate endlessly whether the Taiping genuinely constituted a Christian sect. The Zei qing huizuan 賊情彙纂, or 'Compendium of Rebel Intelligence', a report compiled for loyalist general Zeng Guofan in 1854/5, alleged that the God-Worshipping Society was but one name given to a southern secret society also known, among other names, as the Tiandihui 天地會 (lit. 'Heaven and Earth Society') and the Tiandihui 添弟會 (lit. 'Increasing (Younger) Brothers Society'), and which were organised by followers of Catholicism. It was not until Hong Rengan, who had spent 7 years with missionaries at Hong Kong, became Taiping prime minister in 1859 that the Taiping began to refer to Western Christians as 'brothers and sisters', acknowledging them as fellow believers, but even then not explicitly professing to be part of a Christian sub-sect.
An older school of thought to some extent regarded the religion of the Taiping as essentially lying on a sort of spectrum from Christianity to Confucianism, and its position on that spectrum could be essentially pinpointed by measuring the degree of Christian versus Confucian influence. Eugene P. Boardman in his 1952 book Christian Influence upon the Ideology of the Taiping Rebellion actively rejected the notion that the Taiping were Christian beyond superficial proclamations, while Vincent Shih spent a quarter of his 1967 monograph The Taiping Ideology detailing the Confucian and classical antecedents to Taiping beliefs and practices. Rudolf Wagner's 1984 riposte, Reenacting the Heavenly Vision, conversely argued that the Taiping were, intentionally or otherwise, part of a broader tradition of revolutionary 'revivalist' Christianity, with Christianity providing an avenue for radical political action that the Confucian framework did not.
However, more modern perspectives generally argue that syncretism is not a pick-and-mix, and that different influences operate in different ways. Jonathan Spence's God's Chinese Son (1996), Thomas H. Reilly's The Taiping Heavenly Kingdom (2004) and Carl A. Kilcourse's Taiping Theology (2016) essentially concur on the idea that the core of Taiping belief was derived from the Protestant Bible, but that their Biblical interpretation was fundamentally shaped by indigenous religious traditions. Spence specifically homes in on Buddhist eschatological pamphleteering, Reilly on the Taiping reimagining of pre-Confucian theology, and Kilcourse looks more holistically at the range of influences behind Taiping belief. Reilly and Kilcourse broadly concur on the idea that Hong Xiuquan saw the classical canon and the Bible as in a sense mutually supporting. However, Hong was not exactly carrying the entire Confucian canon over to his system – and it's this that I'd like to explain a little further.
While Hong Xiuquan openly rejected Confucianism, the modern scholarly consensus is that he nevertheless built heavily on the Confucian classics. So how was he able to do this? To answer this we need to look at what exactly the Confucian canon consisted of. The core texts were, of course, the Four Books and Five Classics (si shu wu jing 四書五經), and there is a reason that they are not collectively called the Nine Texts or something similar. To use a slightly clumsy analogy, the Four Books were like secondary sources in that they were read as commentaries upon the Five Classics, which might be considered similar to primary sources in that regard. While the Old Text and New Text schools disputed whether Confucius collated or composed the Classics, respectively, the Lunyu 論語 (Analects) was essentially an illustrative commentary upon an existing philosophical tradition, and the other books, the Daxue 大學 (Great Learning), Zhongyong 中庸 (Doctrine of the Mean) and Mengzi 孟子 (Mencius), were expansions upon that interpretative framework. It was theoretically possible, however, to reject that Confucian interpretative layer without discarding the core texts. As such, it was possible for the Taiping to reference, without much mental gymnastics, the Yjing 易經 (Book of Changes), and for their manifesto for societal organisation, the Tianchao tianmu zhidao 天朝田畝知道 (Land System of the Heavenly Dynasty), to be essentially a straight copy, slightly reworded, of parts of the Zhou li 周禮 (Rites of Zhou), a sister text to the Liji 禮記 (Book of Rites).
How did this play into Hong's interpretation of the Bible? I'll highlight two elements here. The first follows on from the Land System, where Biblical notions reinforced and was itself reinforced by the Rites of Zhou's legendary system. Both the Rites of Zhou and the Land System of the Heavenly Dynasty proposed that all should receive an equal share of land, and that all households would provide one man for military service, and that officers, promoted and demoted by merit at regular intervals, also performed civil bureaucratic functions, such that a distinction between the literary sphere (wen 文) and military sphere (wu 武) was not drawn. Collectivisation could find precedence in New Testament ideas of charity and the community of believers, while the militarisation of the state and people could find justification in the conquest narratives of the Old Testament.
The other element regards Hong's political philosophy, which is the focus of Reilly's monograph. The enigmatic deity Shangdi 上帝 (lit. 'High Sovereign'), head of the Zhou era pantheon, appears several times in the Classics, and conveniently the term shangdi was used by Protestant translators for 'God'. Hong thus came to see the pre-Imperial deity of Shangdi and the Abrahamic God as one and the same, and regarded Confucius as having been a key figure in supplanting Shangdi with a pantheon of lesser deities, while Qin Shi Huang, by taking the title of huangdi 皇帝 (Emperor) sealed the deal by usurping the divine title of di 帝. All subsequent emperors, in Hong's worldview, had continued this pattern of usurpation, and the Qing were no different. What Rudolf Wagner regarded as a Christian 'revivalist' movement could more accurately be called a Chinese classical 'revivalist' movement in the sense of reverting to an older, purer model of society based on an unfiltered reading of the Classics, but with the insertion of the Bible among them as the central text.