I advise you to look into the Second Great Awakening -- a more-or-less US Protestant-specific movement reacting against the rationalistic/deistic movement, which led to a spread of and rapid diversification of protestant, often Calvinist/Arminian theology. One of the hallmarks of the era was missionary activity -- churches like Seventh-Day Adventism, the Latter Day Saints (commonly called Mormons), and several others that we associate with a strong missionary movement have their roots in this movement. Funds and volunteers raised to send out missionaries as part of "benevolent societies" was a common theme, and Asia and the Pacific were a major target of such groups, in large part due to the large number of people there who were not already exposed to organized European missionary activity, as was the case in, say, Latin America. Understand, of course, that I do not propose here that there were not also missions to "save" Catholics and other Christians in other parts of the world; I simply point out why East Asia in particular (China and Korea come especially to mind) had such a strong influx from these groups.

Latourette did a great deal of early-20th century writing on Christianity in East Asia, purely as a starting point.