I would argue that it isn't simply a clear-cut case of some Taiping followers doing so for religious reasons, and others for ethnic ones. Rather, these two aspects of Taiping ideology were closely linked.

It must be granted that we do not hear much of the Taiping in their own words. Official publications were not produced by rank-and-file members, and while there was some Qing interest in what the rebels did, at the very least that doesn't tend to get brought up in a lot of secondary material. But there are some sources that give an indication as to what Taiping adherents prioritised, most notably the writings of Stanislas de Clavelin, a French missionary who recorded a conversation (translated into French) with an old Guangxi convert in 1853. It is rather lengthy, but the key passage (translated into English by Prescott Clarke) is here, where the Taiping soldier describes their long-term plans for the Manchus:

Finally, concerning the Tartars, when we consider the evils that they have caused us, and the abasement to which China has sunk under their government, one cannot dream of entering into an agreement with them; let them return to graze their flocks, or else prepare themselves for a war of extermination. And besides they are idolators, incorrigible idolators. Would the Heavenly Father forgive us for thus forgiving them?

This passage is pretty clear that anti-Qing sentiment was twofold. Firstly, we see an ethno-cultural antipathy towards the Manchus, who are here lumped in generically in a continuum of nomadic 'barbarians', both through the use, presumably, of some term like Dada 韃靼 ('Tatar') or a derivative, and by alluding to the Manchus as being principally herders, when they were in fact sedentary agriculturalists. Secondly, there is a religious objection: the Manchus were 'idolators', and it was God's will that they therefore be destroyed or converted – and to this Taiping soldier, conversion was not going to be possible. Neither of these elements necessarily takes precedence (albeit in the context of a wider conversation which centres on religion).

But I think this leads us into the important matter of how we frame Taiping ideology. We tend to assess the Taiping in terms of whether or not, or to what extent, they were 'Christian', which takes a very essentialist view of Christianity as a concept: if you believe certain things and not others, then you are or are not a Christian. But the reality is more complex. Carl Kilcourse, whose Taiping Theology is the best book on Taiping religion out there, is in my view right to highlight that there was genuine devotion among the Taiping to a creed that was directly modelled on both Testaments of the Bible, but his consequent identification of them as 'Christian' is at least semantically problematic, because it is not entirely clear that the Taiping self-identified as being part of a wider global religious movement, outside of a small handful of better-connected leaders like Hong Rengan.

Rather, the Taiping are probably best understood, as argued by Thomas H Reilly, as a movement focussed principally on the restoration of a presumed 'pure', monotheistic China believed to have predated the Warring States period. The frequent appearance of Shangdi 上帝 ('High Sovereign', also the Protestant translation of 'God') in the proto-Confucian canon (particularly the Five Classics – the Book of Changes, Poetry, Rites, Documents, and the Spring and Autumn Annals) was believed to indicate that Confucius and his successors had come to erase God and establish in his place a false pantheon, which in turn came to legitimise the rule of Qin Shi Huang, whose title of Huangdi 皇帝 ('Emperor') usurped the majesty of the now-deposed Shangdi. With Shangdi being the God of the Bible as well, the Taiping official line – which evidently the soldier that Clavelin interviewed believed in – was that the Bible and the Five Classics formed a mutually complementary, 'pure' canon independent of Confucian corruption. In this vein, the Qing could be understood in one sense as like any other previous empire: an institution founded on blasphemy and the denial of God. At the same time, this vision of an originally pure China also meant that the Qing, as a foreign-established conquest state, constituted an ill in their own right by virtue of their barbarous nature and domination of China.

But hardline ideological devotees did not constitute all of the Taiping army, nor, at many points, its majority. Rather, sides were chosen in large part, it seems, based on local and personal loyalties tying a comparatively disinterested peasantry to charismatic and politically interested local leaders. Granted, the focus of research in this area has mainly been where the sources lie: the local elites and other community leaders, who wrote or were written about. For instance, in Zhuji County in the province of Zhejiang, He Wenqing, the patriarch of the He clan, established a militia which joined the Taiping in the spring of 1861, and convinced the strongmen of five other counties to defect as well; while in Shaoxing Prefecture, a local messianic Buddhist cult leader named Bao Lisheng assembled some 2000 militiamen in support of the Qing. These leaders chose sides from a mixture of ideological conviction and pragmatic considerations, but their ability to mobilise manpower towards either side of the war effort seems linked not to grander ideological convictions on the part of their followers, but rather to more direct local and personal ties: both leaders first secured the support of their immediate clan and, then broader lineage connections, then the wider community, and eventually established multi-county connections. In many ways, this was, in microcosm, what Zeng Guofan was able to do in his establishment of the Hunan Army: get his brothers, colleagues, proteges and other close associates together, and then send them out to make connections with leading figures in the gentry, who would in turn get together established community leaders, who could in turn recruit troops from among their communities. We know little about these individuals (so far), but in general terms, it seems that they were not motivated specifically to support the Qing or the Taiping, but rather rallied behind existing community leaders.