Neither Sun nor Chiang were strong Christians, Sun only put up a baptization ritual and a Christian funerary ritual to show gratitude to the Christians who helped him. Sun mentioned this clearly in his work 伦敦蒙难记 where he said that
"my confidence in Jesus' teaching became weak after studying science, when I was in school, I often felt the irrationality of Jesus, because I was not satisfied I started reading philosophy books. At the time, my believes lean towards evolution, but I haven't completely abandoned the teaching of Jesus." 予于耶稣教之信心,随研究科学而薄弱。予在学校时,颇感耶稣之不合理,因不安于心,遂至翻阅哲学书籍。当时予之所信,大倾于进化论, 亦未完全将耶稣教弃置也
Sun's wife Song Qingling later wrote that Sun told her he did not really believe in God.
Sun did stress the importance of religion, but evidence shows that it was Buddhism, not Christianity which he felt was the religion that China needed.
According to Sun's grand daughter: Sun told her "Buddhism is the salvation of the world, Buddhist studies is the mother of philosophy, the people cannot lack a religion...Buddhism can cover the shortcomings of law" 祖父曾讲:佛教是救世之仁,佛學是哲學之母,人民不可無宗教之思想,盖教有辅政之功,政有护教之力,政以治心,相得益彰,互不悖逆,总括来说,佛教补法律之不足。」(孙穗芳,我的祖父孙中山下册,台北禾马文化,1995年,页286
If the Republic of China remained, chances are Buddhism, especially Tibetan Buddhism would become the dominant religion in China. The efforts of the Panchan lama and Dai Jitao converted hundreds of thousands of Chinese (coupled with the Buddhist revival movement and the Buddhist influenced New Confucianism growing in China itself). This was in fact one of the major reason that Tibet chose to accept the PRC demands and turned down the CIA's offer to join the anti-communist bloc because they thought, from Panchan's writings, that the ROC was on the verge of becoming Tibetan Buddhist.
Even today, Tibetan Buddhism is growing in China, converting even some party members.
Christianity only sees more growth among rural and suburban areas. You can also bet that the Communist party will rather see more Tibetan Buddhism converts than Christianity.