What were some reactions from the European Christian community to the Taiping Rebellion and their brand of Christianity?

TW: Mentions of self-harm, suicide, and murder

An answer to this question can be, generally, divided into two perspectives: yes, and no.

Well, it's a bit more complicated than that in terms of the actual basis for supporting or opposing the Taiping, but, well, those are basically the two possible outcomes. There are many reasons, operating on many layers, for why missionaries were not of one mind about the Taiping. One such reason was denominational: outside of a brief period up to early 1853, Catholics were broadly hostile to the Taiping, informed in part by inadvertent persecutions of Catholic civilians. That does not, however, mean that Protestants were broadly supportive, as another factor was interpretative, and revolved in large part around a fundamental question of Taiping Christology, and the status of Hong Xiuquan: namely, was Hong's claim to be the son of God literal or figurative?

If the claim was literal, then that was very obviously and fundamentally blasphemous, and usurped the divinity of Christ, something that most good Christians would never countenance. I don't know that there are any Europeans who ever took such a claim to divinity seriously. If the claim was figurative, however, then Hong was using 'son of God' in a sense that was much more palatable, in that he was simply reaching a perfectly logical conclusion to the notion that all people are God's children. Although, many figurativists did suggest that Hong's use of 'son of God' as a noteworthy title emerged out of a failure to recognise the divinity of Christ, and interpreting 'son of God' as being an appellation for anyone whom God had specifically empowered to carry out his will on Earth, which included Jesus in the past and Hong in the present. As a result, not all figurativists were pro-Taiping either right off the bat. Anyone in prolonged contact tended to lose sympathy relatively quickly. At some stage, defences of the Taiping became one of arbitrary personal preference on the part of a select few who could reconcile their faith in the Taiping cause with their actual religious faith. And, it is worth noting, few of those were missionaries.

I've written a few answers covering this in the past (check out my profile for more of these), but for me the most interesting example to dig into for this answer is Issachar Jacox Roberts, whose connection to the Taiping was particularly personal, because he had, for a few months in 1847, been Hong Xiuquan's teacher. Roberts' background was somewhat ignominious: born in Tennessee, he chose to proselytise in China almost on a whim, and 'paid' for his passage to Canton in 1837 with a donation of land to the Baptist Council that turned out to be a bunch of gravel pits. After years of exaggerating the size of his congregation and padding out the donations from his own pocket, he was finally excluded from the communion by the Baptist Council in 1850 when, after his colleague and housemate Rev. James Bridgman attempted to take his own life by cutting his throat with a razor, he refused to come to his aid, declaring, in a note sent in response to the news, 'Let the dead bury their dead, but I must preach the gospel.' Basically, not the most pleasant character if one were to be totally honest.

Roberts' early writings on the Taiping were almost uniformly laudatory, even before he was aware there would be an insurrection: on 27 March 1847, he wrote a letter to an associate in Tennessee, William Buck, on the matter of his first meeting Hong Xiuquan and his cousin Hong Rengan, stating that

These are the first inquirers I have had this year and their experience thus far as stated by themselves affords the most satisfaction of any Chinese experience I have ever heard for the length of time. I trust that this may be the commencement of the outpouring of the Holy Spirit upon this benighted people. May the Lord add to the church here daily such as shall be saved and to His name be the glory forever.

Roberts also states in this letter that he considered translating the written narrative he was offered by the Hong cousins, but this seems never to have transpired, nor does he seem to have preserved the originals, sadly. Still, as you can see, it's a pretty positive outlook.

As news of the ongoing rebellion in China started becoming more widespread, Roberts wrote again on 6 October 1852, this time to the London-based Chinese and General Missionary Gleaner, prompted by the sudden arrival of Hong Rengan in Hong Kong as a refugee. Roberts had evidently obtained a copy of Hong Rengan's personal testimony of preceding events (this, or another original, is housed in the collection of the Cambridge University Library), as his piece, titled 'The Chinese Struggle', spends its first part translating said narrative, followed by his own commentary. Yet again, this commentary was quite positive:

This is China's crisis – how earnestly ought christians [sic] to pray and strive for the furtherance of the gospel among these people under present circumstances; and to make the most of every opportunity of usefulness that may soon offer!

And he ends his letter with a postscript ecstatic at the prospect of the Taiping becoming a bridgehead for missionary efforts, with the Taiping providing the initial mass conversion, and the missionaries offering their services to correct their apparent theological mistakes, converting all of China seemingly overnight.

Roberts held onto this hope, and in 1853 he almost went to Nanjing himself on invitation from Hong Xiuquan, until informed by the American consul that he would be committing a capital crime by violating the US' neutrality in the ongoing conflict.

Still, Roberts was undeterred in his support. Writing in June 1854 in a piece titled 'Grand Plan for Missionary Increase in China', published in The Primitive Church Magazine in London in January 1855, Roberts confidently predicted an imminent Taiping victory and the doors to China being kicked wide open for missionary proselytisers. He also proposed an organisational structure for a nondenominational, multinational mission council based in China itself, which to be honest reads as an obvious jab at the Baptist Board in Boston which had excluded him. Advantage 2 that he cites is that 'should any little understanding arise among the missionaries, they could be promptly heard by the committee', which is, to be frank, really quite rich coming from a guy whose most notable 'little understanding' was refusing to come to the aid of a colleague who had just attempted to take his own life.

Roberts' grand plan for a global missionary revitalisation never quite materialised, but he again wrote a laudatory piece in 1856, titled 'Tae-Ping-Wang', for publication in New York-based Putnam's Monthly Magazine. 'Tae-Ping-Wang' is a bit of an odd piece, it's a biographical sketch of Hong Xiuquan up to 1851, evidently based on a mix of Hong Rengan's direct testimony and on Theodore Hamberg's The Visions of Hung Siu-Tshuen, but it shows a continued commitment to the cause.

By late 1860, Roberts was in the Shanghai area, and wrote a series of letters, published in consecutive issues of the Hong Kong-based Overland China Mail, reporting meetings with important Taiping leaders, and pushing strongly for foreign recognition. On 9 October (published 29 November), he reported a productive meeting with the Loyal King, Li Xiucheng, and concluded his letter with the repeated refrain, 'why not make a treaty'? By 12 November, he was in Nanjing as a guest of Hong Rengan, now the Taiping prime minister, and had his first audience with his other former – and he hoped, soon-to-be-once-again – pupil. His tone in this letter is a bit more uncertain than before: he reports Hong's main interest being theology, which, 'I must acknowledge, was not very correct', and notes that the audience was interrupted a few times by Hong ordering his officers to perform obeisance, but 'In this I took no part.'

Still, Roberts believed in the cause throughout 1861, writing several pieces extolling the Taiping and condemning those in the European powers who would try to prevent their righteous victory over 'the devilish imps' of the Manchu state. His personal reservations were increasingly coming through in more private correspondence: British consular correspondence in May reported a certain disgruntlement on his part about being duped into kneeling before Hong at one stage. Yet his public zeal seemed undiminished. Writing to the China Mail in July, Roberts asserted that the Taiping were God's vengeance against the idolatrous Qing, and asked, coldly and more than a little bloodthirstily,

...would it not be better in the highest sense of the word for half the nation to be exterminated, than to go on as they have been doing, if the other half would thereby learn righteousness!