It is true that the Ma Clique had a strong affiliation with Islam, being that many were ethnic Hui, most of whom practice Islam, which is still present in modern China. Because of that, I think the Ma Clique gets a lot of attention from Western observers, many of whom are usually surprised Islam is present at all in China. Anyone who has studied China, however, knows that while East Asian religion can be nebulous and extremely different from Western religion, "China" as a whole is quite ethnically and religiously diverse.

During the 1920s, it became somewhat of a fad among various Warlords to convert to Christianity. Its really hard to pinpoint something like a persons attachment to cultural and material things, so I cant really just go around saying "Well, warlord X converted to Christianity due to Y," but there's a good indicator that the powerful presence of the YMCA in China during this time was a driving factor. At a time when China was a failed state, various Western (and/or Christian) institutions that had existed within China for decades still held tremendous financial and political influence. In addition, Western-styles of life, politics, and culture seemed to be usurping traditional Chinese culture and power since the Opium Wars. Thus, for many wealthy elites, it made sense for them to send their children to Christian-run private schools in China, or to Japan, where they could receive a good, but more importantly stable, education. As a result, many of these future Chinese leaders grew up to be Christian. Again, this begs the question just how spiritually affiliated with their religion they were, who knows, but there were certain political implications to publicly announcing your new found Christian faith.

Thus, the Zhili Clique ended up having quite a few warlords who identified as Christian, most notably Feng Yuxiang, who later became a close affiliate of Chiang Kai-shek, who was also Christian. Many elites within the Kuomintang were well known Christians, including the Soong family, who consisted of future Minister of Finance TV Soong, Chiang's wife Soong Mayling, and Soong Ching-ling who married Sun Yat-sen and later became a communist revolutionary to her family's dismay. So while Christianity didn't peak among most warlords during the pre-Nanjing period, it as present and became more powerful with the rise of Sun Yat-sen and Chiang. It proved to be a powerful tool, along with Soong Mayling's American education, in securing trust and support from Western powers against Japan, which realistically didn't amount to much.

This religious attachment, and additionally attachment to a foreign ideology, became quite the bullet in the butt once communism became popular among Chinese elites. One can point out the irony of a party following a Western-born political ideology (communism) pointing the finger at KMT officials for adhering to a foreign religion, but I digress. Still, while I'm not expert on Chinese elite's religion, I imagine that most of these men were not exactly attached to Christianity the same way that Western leaders were. I do know that we have access to most of Chiang Kai-shek's personal diaries, and he was quite devout to Christianity. Additionally, he often blended many elements of Christianity with Confucianism, so we can possibly assume other devout Christian elites did too, as is natural with religious expansion.

So in conclusion, I can't give a straight answer, but we do know that Christianity became powerful during the 1920-30s among elites, especially Republican politicians. Christianity quickly fell out of favor once the CCP took over, but to this day roughly 2-3% of Chinese identify as Christians, which is about anywhere from 30-50,000,000 people.