 Following the May Thirtieth Incident of 1925, in which British-led troops killed unarmed student demonstrators in Shanghai, Cheng was forced to resign from his teaching post.For the next two years, he served as a chaplain in the army of Feng Yuxiang, known as "the Christian General".When Feng aligned with the revolutionary Nationalist Party of Chiang Kai-shek, Cheng resigned from the chaplaincy.