One thing I haven't seen very often is TL's where Christianity is much more powerful in Asia. What would be the best scenario to get us there - say where 30% of the population of Asia at least was Christian, along with at least one major state?
As I see it there were three chances the the Church of the East had:
1. Early in its history, the Church of the East was making great strides forward in Persia. However, its perceived status as a foreign, Roman religion held it back. Indeed, it seems the embrace of Nestorianism by the Church was in part for political purposes - to clearly delineate it as being beyond the control of Rome. However, it still suffered periodic crackdowns which stopped its embrace by the majority of the population (although it may have been the majority religion in Mesopotamia). The rise of Islam, of course, essentially cut off the chances of the Church there to expand, as Islam won far more converts. But without Islam, would their time have come?
2. Even after the rise of Islam, the Church flowered in China under the Tang Dynasty, and was prominent for roughly 200 years. The Church was somewhat limited, once again, due to its perception as foreign (for some time, Chinese were only allowed in the lowest ranks of monkhood). The suppression of Emperor Wuzong in 845 (who also attacked Buddhism and Zoroastrianism as foreign) effectively ended the Church until the Mongols re-introduced it. While it seems unlikely that a Tang emperor would convert to Christianity, one wonders if without suppression the Nestorian community could have grown at least to the strength of the Hui in present-day China.
3. The final Nestorian flowering was under the Mongols, which began converting to Christianity in the 7th century. The wives of Genghis Khan's sons, for example, were Christian, and his grandson, Mongke, while not openly Christian, told ambassadors from the west he privately wanted the Mongols to worship the Messiah, but he could not upend the mongol tradition of religious tolerance. The Yuan Dynasty, when it conquered northern China, re-introduced Christianity, but it was again expunged by the Ming. Still, in some ways the Mongols seem hemmed in - most of their foot-soldiers, outside of the far East, were already Muslim, and thus incredibly difficult to proselytize to.
Anyway, which of the three strikes you as the most interesting place to insert a POD?