Right now Christians make up only 3.9% of the population of Taiwan--and of them, two-thirds are Protestants.
https://en.wikipedia.org/wiki/Religion_in_Taiwan#Christianity I don't see why the percent of Catholics would be
that much higher on the mainland. Yes, Catholics are about 11 percent of the population in South Korea, but there are distinctive historical reason for this which I am not sure would apply to China.
https://www.aljazeera.com/indepth/o...icism-important-korea-201481717037383818.html
That's part of it in Korea, but for Christianity in general, it goes deeper. Christians, especially American Protestants, almost entirely sided with Korean independence activists during the March 1st Movement (whose 100th anniversary was this month), one of the most important events in the history of Korean national identity. When the Japanese crushed the activists, American missionaries basically disseminated the Korean movement to the rest of the world, in particular America. So mainstream American (like the kind of person who would staff the State Department) opinion was fairly negative towards Japanese imperialism already back in 1919, and thus Korean nationalists have always had a sweet spot for Christians. US enmity towards Japan in 1940/1941 didn't come out of nowhere - it had been building for decades.
https://providencemag.com/2016/07/christianity-korean-independence-movement-1895-1945/
Edit: On that note, a common alternate history trope is that a Republican (Alf Landon?) USA would have been isolationist and avoided World War II, but with WASP America deeply hostile to Japanese imperialism and sympathetic to China/Korea, they might have not helped out the UK/USSR as much against Germany, but they'd still have kept the pressure on Japan, which probably leads to America entering the war anyways.