I briefly flirted with the idea of writing about Qing attempts at syncretising war deities, something that isn't my main area, when it suddenly and embarrassingly struck me that my primary specialism revolves entirely around a syncretic religion! Taiping Christianity is something I've written on a few times before, but hey – why not do it again? Trying to talk about Taiping Christianity in this context is made complicated simply by virtue of how much has been written on it, and therefore how much material you have to try to select from and summarise, so instead I'll be mostly historiographical here, and if you want more specifics then have a gander through my past answers on the subject.

To lay out the groundwork first, Taiping Christianity was a short-lived syncretism of largely Protestant Christianity and Chinese popular religion that first emerged in 1843 and which would be effectively destroyed by the Qing government in 1866. Its founder, Hong Xiuquan, believed that he had received visions in 1837 which, when interpreted through the information found in a Protestant missionary tract which he read in 1843, identified him as God's second son, with a mission to drive out the demons that infested the earth and to restore the worship of the true God. He and a cousin, Feng Yunshan, then established the God-Worshipping Society in rural Guangxi, which served as a militia and aid organisation as well as a church amid a general weakening of Qing local government in rural areas. The Society increasingly came to blows with the Qing before formally declaring the establishment of an independent state, the Taiping Heavenly Kingdom, in 1851, which relocated to Nanjing over the course of 1851-3, from which it expanded to control much of the Lower Yangtze valley, before being eventually ground down and destroyed by the Qing.

The conflict sometimes known as the Taiping Rebellion, the Taiping Civil War, or the Taiping War, was perhaps the most momentous event in China's 19th century history, causing millions of casualties, ravaging some of China's most prosperous regions, setting off a number of smaller regional uprisings, and, perhaps, emboldening a somewhat dormant but nonetheless potent strain of Han Chinese nativism that ultimately led to the overthrow of the Manchu-led Qing empire in 1912. The image of the Taiping either as nationalist heroes or as social revolutionaries (or occasionally as both) would be mobilised by anti-Qing agitators and post-Qing regimes, with Sun Yat-Sen and Mao Zedong especially being proponents of the idea that their own political movements were outgrowths of the Taiping's failed revolt. From an external perspective, the ostensible oddity of the Taiping's religion became an object of curiosity for generations of Anglophone scholars.

But it may be surprising to learn that seriously approaching the Taiping in syncretic terms is comparatively recent. A lot of mid-20th century scholarship on the Taiping tends to adopt a highly essentialist and binary viewpoint in which the Taiping were either Christian or not. Eugene Boardman in 1952 declared the Taiping non-Christian because of their failure to adopt what he (as a Quaker) saw as key tenets of Christianity such as the Golden Rule; in 1962 Joseph Levenson declared that the Taiping were sincere in their Christianity and thus presented a fundamental challenge to the Confucian order; in 1967 Vincent Shih dissected a vast number of Taiping official documents to unearth their influences, and concluded that the Taiping were broadly Confucian in their ideological influences, and merely adopted Christian trappings as a pretext for an essentially secular agenda; in 1973 Jen Yu-Wen asserted that Christianity was fundamental to the Taiping as a 'revolutionary movement'. Rudolf Wagner would be one of the first to seriously approach the Taiping through a syncretic lens with Re-enacting the Heavenly Vision in 1982, stressing the importance of the intersection between Christianity and Chinese folk religion in the coalescence of Taiping ideology. Jonathan Spence, in his narrative history of the Taiping, God's Chinese Son, in 1996, would specifically hone in on the probable influence of Buddhist eschatological pamphlets on Hong's interpretation of Christianity.

The two modern touchstones for any serious discussion of Taiping religion are Thomas Reilly's The Taiping Heavenly Kingdom: Religion and the Blasphemy of Empire (2004) and Carl Kilcourse's Taiping Theology (2017). Each work approaches the subject somewhat differently and homes in on different aspects. Reilly situates the Taiping principally in chronological context, focussing especially on the problem of communicating Christian religious concepts in written Chinese, and on how the Taiping's approach differed from – but also evolved out of – those of the Catholic and later Protestant missions in China. In turn, a major focus is placed on the Taiping idea that China had once practiced a form of Abrahamic monotheism before the introduction of Confucianism and latterly Buddhism, thus making Taiping Christianity in some ways a fundamentally nativist religious movement. Kilcourse situates the Taiping in a global rather than chronological context, arguing that the Taiping should be treated as a legitimate 19th century 'glocalisation' of Christianity, that is to say that while Christianity was a global religion, the Taiping imposed their own imagery and meaning onto it. He doesn't reject Reilly's work in so doing – indeed, the Taiping's Sinocentric view of Christianity is a critical element of this 'glocalisation' – but he arrives at these conclusions through a somewhat different framework.

I think the area of Taiping syncretism that is most apparent and pervasive in their writings and history is in how Christianity was interpreted in a context where interaction with spirits, both good and evil, was commonplace. Hong's alleged mission to destroy demons and expel evil spirits was not spun from whole cloth, but specifically emerged out of a popular understanding that spirits and demons actively involved themselves in the world of the living. Texts discussing Taiping activities, especially those of the pre-Heavenly Kingdom years, have a particular focus on interactions with such spirits. I go into much more detail in this answer, but we see this for instance in an episode supposed to have taken place in 1847 in which Hong Xiuquan personally carried out an elaborate exorcism against a local deity known as King Gan, a malevolent entity whose image, housed in a cave, was given offerings as a form of appeasement. What this illustrates particularly pertinently is that the Taiping's rejection of spirit-worship derived from belief and not scepticism: they did not reject the existence of spirits and other lesser deities, but were in fact profoundly convinced of their existence, while abhorring the existing religious practices surrounding them. Equally important was the act of spirit channelling, in which a human claimed to serve temporarily as the vessel for a spiritual being. While Hong Xiuquan and Feng Yunshan ultimately rejected most such channellers as illegitimate, they did ultimately recognise the claims of Yang Xiuqing and Xiao Chaogui to be able to channel God and Jesus respectively (note that the Taiping adopted a sort of unitarian Christology), such that for several years (until Xiao's death in battle in 1853 and Yang's purging and execution in 1856) the Taiping could claim to be receiving continuous revelation from God and Jesus – the sole deistic presences worthy of worship – through these mortal vessels.

Again, the above is only one example of Taiping syncretism in practice, and there is plenty more out there to discuss. Aside from my own answers you may wish to go straight to the sources, including:

• Rudolf Wagner, Re-enacting the Heavenly Vision (1982)

• Jonathan Spence, God's Chinese Son (1996)

• Thomas Reilly, The Taiping Heavenly Kingdom (2004)

• Carl Kilcourse, Taiping Theology (2017)

• Daniel H. Bays, 'Christianity and the Chinese Sectarian Tradition' (1982)

• Roland Boer, 'Marxism, Religion and the Taiping Revolution' (2016)

• Carl Kilcourse, 'Instructing the Heavenly King: Joseph Edkins’s Mission to Correct the Theology of Hong Xiuquan' (2019)

This is some limited evidence of syncretic religious practices in Anglo-Saxon England.

Anglo-Saxon England was at least dominated by Germanic pagans from the late 400's to the 600's AD. From the Augustinian mission arriving in Kent to the destruction of the last pagan kingdom on the Isle of Wight, according to Bede, a little less than 100 years passed.

Bede tells us, and he is really our only textual source, that relapses into paganism were relatively common. The most well known example of this was the king of East Anglia, Rædwald, who according to Bede was one of Bretwaldas, wide rulers, of the Anglo-Saxon kingdoms. He supposedly relapsed into paganism at the behest of his pagan wife and to his dying day maintained a temple to the pagan gods with a small altar also dedicated to the Christian God. While an interesting story on it's own it is also backed up with archaeological evidence. The famous Sutton Hoo royal burial is commonly attributed to Rædwald and contains evidence of this syncretism that Bede describes. Interred within the ship burial were not only weapons and armor, but food stuffs, animals, and in a break with the rather pagan nature of the rest of the burial, two silver spoons labeled with the names of Paul and Saul. Which are not exactly Anglo-Saxon names or Gods.

However we should not mistakenly assume that this held through all of Anglo-Saxon England. Some pagan kingdoms such as the Isle of Wight were supposedly entirely exterminated by their Christian neighbors, but it was rather common for there to be some practices and sites that are believed to reflect pre-conversion practices. Some practices such as leaving out grain for local spirits, wearing amulets to ward off disease, and more were continued after conversion, to the vexation of some Christian authorities. Other beliefs such as witches and magicians likewise persisted, despite hostility from Christian intellectual culture.

In the end, a variety of pagan practices were extensively repressed by Christian authorities, both the Church and King. Practices such as horse consumption, infanticide, sacrifice, and so on were all strenuously repressed as incompatible practices. Syncretism could only go so far, and after a certain point adherence to orthodox religious practice was necessary.

So while syncretic practices inevitably arose and were in some instances tacitly approved of by Church authorities, this only went so far. Indeed Christian authorities spent a great deal of energy trying to determine what was pagan practice and what was harmless. This period though only lasted for a relatively short period of time in most areas. Saxony and England were converted relatively quickly, in the span of a century or so.