Christianity was present in earlier times, but really started to be seen as something that fits into Chinese society after the Xinhai Revolution. Many Republican intellectuals had some passing experience with Christianity or professed Christianity themselves, and strains of Protestantism are now a major force in Taiwan and somewhat underground in the PRC. I think that Christianity could become a major force in China earlier than OTL if it ties itself to a successful political/social movement, and the Taiping lend credence to this in a somewhat roundabout way. However, this change may need to be extremely drastic-- conversion rates in Korea only rose dramatically after the upheavals of division and the Korean War, when churches (which had established some minority communities of converts before and during the Japanese colonial era) earned a positive image in society by doing charity work and keeping communities together. American influence also undoubtedly helped things along. A China that undergoes a similar level of upheaval might end up 25% Christian in the end, but probably won't be such a fun place to live. Then again, OTL China did not have it easy either...
Failing that, just have more missionaries enter China in this time, maybe under a Qing interested in education and medicine (where missionaries could make immediate and helpful contributions). As they carry on with their work, they will earn converts, and those converts then spread the word.