Talking about a 'purge' happening after the rebellion sounds like you're envisioning a widespread religious movement being stamped out after the rebellion. But the movement (in decline for quite some time by then) collapsed entirely with the death of Hong Xiuquan in April 1864 and the fall of Nanjing in July, where hundreds of thousands of the remaining Taiping fought to the death or committed suicide.

The Taiping were defeated with the help of western powers, to some extent directly (in particular when Shanghai was threatened) but mainly through the Ever Victorious Army, trained and led by the British, and their French counterpart, the Ever Triumphant Army. Obviously these forces would not fight Christians; these same western powers had demanded and received the right for Christian missionaries to work freely in China, as well as special legal protections for them, in the Unequal treaties. Even if they had wanted to (although I've not seen anything to indicate that they did, at that time), the Qing government could not use the Taiping rebellion as an excuse for purging Christian missionaries, or let them come to harm through inaction, because that would surely lead to one or more Western powers intervening militarily and forcing restitution and another Unequal Treaty with more concessions, as occurred later with the Boxer Rebellion.

I haven't seen anything indicating the Chinese, or at least knowlegeable ones, were under the impression the Taiping were Christians in the same vein as western missionaries and their converts. While some westerners initially held hopes for the Taiping and their state as a start of a new Christian China, that disappeared as more information about their idiosyncratic faith reached the west, as well as information about atrocities they'd committed.