Basically, missionaries saw China as a big score. They could go there and save the souls of hundreds of millions of people. Particularly, British missionaries organized chiefly by the London Missionary Society traveled in droves to China (tens of thousands of them in the 19th century) to convert the population. They did so mostly through humanitarian and community projects. Humorously, this produced a huge population of what are referred to as 'rice Christians,' who would claim to be Christian only to receive the food offered in missions. All the same, these missions saw increasing effectiveness, especially with the gradual opening of China after British (and other European) actions during the Opium Wars, the Boxer Rebellion, and the Taiping Rebellion. Missionaries are at first relegated to port cities, but gradual changes allowed them to move inland. The Taiping Rebellion itself was sparked by a man who claimed (perhaps literally) to be the brother of Jesus Christ, and it was instigated at least in part to create a Christian state in China.

The American 'China Lobby' which grew to support the Republic of China in its efforts against the People's Republic was, at least in part, motivated by Chiang Kai-Shek's and Sun Yat-Sen's conversions to Christianity. The two married sisters in a prominent Chinese Christian family, who rose to celebrityhood in America during the war.

It's important to contrast this with the actions of Catholic missionaries, who largely focused their efforts on a top-down conversion of China, focusing on converting officials and, ideally, the emperor himself. They saw comparatively little success, especially after the 16th century Vatican decision forbidding traditional Chinese ancestor worship as a form of idolatry. Nonetheless, Jesuits had a noticeable presence in China for hundreds of years since the founding of Jesuit efforts by (in part) Matteo Ricci in the 17th century, with one French Jesuit famously negotiating the precarious Treaty of Nerchinsk in 1689 between Russia and China, establishing the borders of the states (which would eventually change after a Russian strong-arm after the Second Opium War, a war in which Russia had no involvement, which established the country's modern northeastern border. There have been Catholic efforts in China since at least the 7th century CE, with notable progress being made during Mongol (Yuan) rule.

On the subject of Russians, Orthodox Christians had a long, yet isolated history in China, mostly functioning to convert Han Chinese and Manchurians near the border.

This is all too say that, basically, China has had numerous long-term efforts by Christian missionaries over the course of centuries, and as the country has no meaningful limitations on the practice of religion, the results of those centuries of effort persist. I think it is important to realize that, while China does have on paper, a single party Communist government, it would be fair to say that the number of Christians has always been greater than the number of politically committed communists. You didn't really see the persecution of Christians in Maoist China that you did in the Soviet Union.

This would almost certainly violate the twenty year rule