With any POD, make Nestorian Christianity (or any Christianity, really, I just think Nestorianism has the highest chance of happening) the dominant religion of China.
Wait a century, on its current pace Christianity will become the dominant religion of China in that amount of time.
However, there was a good time when China could have become Catholic. The Ming Dynasty, and especially the Southern Ming, was rapidly being converted to Christianity amongst the nobility and merchant classes, with a significant peasant following. Eventually one of the fleeing would be Emperors and their entire court converted in an effort to secure support from the Portuguese and Spaniards. Have them hold out longer and have either of those two states more interested in keeping the Southern Ming around. All that's needed is a stalemate (something that happens plenty of times in the past) and from there conversion in the south can be completed and insidiously move northward.
A similar event was occurring under the Qing, but the moment a doctrinal dispute took hold between the various christian factions it was doomed. Some said that Confucianism was not incompatible with Christianity and that in all likelihood Confucius was saved. Others said that Confucianism was just as wrong and backward as Buddhism or Taoism and should be eliminated. Both sides appealed to the Pope, and even before the Pope made a decision the Qing court was annoyed. The religion was banned and outlawed by the Chinese Emperor because it challenged his temporal and religious authority. Christianity, which was only just beginning to recover from the fall of the Ming, was crushed once again.
Christianity would have moderate success in the later Qing years, being about as popular as Islam was, or even more so. Sun Yatsen even attended a christian school in china. That is, until the old Empire fell, and the Nationalists and the Communists fought each other. Both frowned pretty heavily upon the religion, and the group faced an existential threat in the form of the Japanese. The civil war left them a disenfranchised minority, and the Communist takeover saw them persecuted during the cultural revolution.
That being said, they've recovered better than most. Depending on who you talk to the numbers of Christians in china are hopelessly conservative (about 15 million) or stupidly excessive (over 100 million). Both numbers are growing at an extremely fast rate.
Three potential things could lead to Christianity becoming dominant much sooner than 2100:
- The Southern Ming Dynasty Holds Firm, if only for several more decades, or the Ming Dynasty remains strong for as long as it can.
- The doctrinal dispute never gets out of hand and both factions work within the Qing government until they are actually in a position to be petty.
- The Qing Dynasty lingers on for a few more decades and the Communist revolution fails.
Nestorian Christianity is more difficult to make a dominant religion, but its possible if you can prevent the backlash against foreign groups and foreign religions near the fall of the Tang Dynasty. Eventually the religion may grow popular enough to gain a patron Emperor of a splinter state and have it spread that way, or you could have it become a religion of the masses through charity, or even possibly wed it to confucianism through schools.