One of the things to keep in mind when dealing with any man who has risen to "greatness" (or one could say, relevance) in history is that nearly, if not all, of these men get there by being pragmatic. A lot of recent historical work touches on this, but sometimes uses stronger language to prove the point. I.e., "X leader is traditionally thought of as a great man because Y, but did you know that he actually did Z, which makes him much more ambiguously evil/good!" Sometimes in academics these types of things are blown out of proportion as someone tries to make a name for themselves as the person who "re-invented how we feel about X." The point is, though, that often times leaders, especially those who are political, must retreat from more zealous positions, or betray certain beliefs in order to compromise towards a greater goal. In Dr Sun's case, this would be a united and modernized China, led by an uplifted, educated, and democratic society, and during his final years, as you pointed out, Sun would have to work with the Soviets, something he was not so keen on doing.
Sun Yat-sen received religious education in Hawaii for most of his adolescence. I've written about the influence of Christianity on KMT members before to an extent, though mainly focusing on Chiang Kai-shek, who was also devoutly Christian. Now, its quite hard to actually interpret how religious an individual is. It can be even tougher when dealing with individuals who are newly converted members of a very alien society to Christianity, a religion which has never really taken ahold of the majority of Chinese society. In fact, its right about the time of Sun's childhood that Chinese society begins viewing Christianity in particularly nefarious lights; Britain has just crushed the entire Chinese empire and Confucian order through some pretty shady diplomacy along with war.
Although Christianity was viewed negatively by the general populace, many wealthy Chinese see the value in sending their kids to missionary schools, or if you're really wealthy, sending them to the US or UK, and if not Europe/USA, then at least Japan. In Sun's case, he would find his way to boarding school at an Anglican institution, the 'Iolani School, and then later going to Oahu College in his later adolescent years. When Sun returned as an adult to China, he would continue medical training at Hong Kong College of Medicine. Throughout his life, Sun would hold close ties to Christian institutions of education. Now, as far as I can tell from what I've read, Sun was absolutely dedicated to Christianity, and Harold Schiffrin argues that Christianity would have a huge impact on Sun his entire life. So its safe to say that Christianity permeated further beyond his younger years. But while Sun stayed in Hong Kong and traveled around China, he grew more interested in politics and less interested in medicine.
So with all that out of the way, lets get into the crux of the question, during Sun's later life as a political figure and not a doctor. In 2020 when we read about or look at monuments dedicated to Sun Yat-sen, he seems like a pretty heroic, larger than life figure, and for many Chinese nationalists he is very much so; perhaps maybe even the only man to stand next to Mao in importance on the mainland, and still a very authoritative figure in Taiwan and abroad, especially among Cantonese ex-pat communities (he has a statue in San Francisco and NYC). But during the 1880s, when Sun's revolutionary campaigns kicked off, he was somewhat of an outcast and a failure. Prominent intellectuals like Kang Youwei and Liang Qichao were negotiating with the Qing dynasty, attempting to reform China in some sort of constitutional monarchy. Sun supported full on revolution. His first planned uprising, in 1895, was a complete failure, forcing him into exile in Japan. It was the first of 10 revolutions, nine which would be failures and one, 1911, which ultimately succeeded. Through all these hard times, there's some evidence Sun turned to the Bible for spiritual guidance, while turning to the Japanese for financial guidance.
Sun's Christian Convictions
We actually don't have a plethora of primary sources on Sun in Japan, unlike Chiang who kept a personal diary and wrote a ton about his spiritual convictions, Sun either didn't keep a personal log or we don't know of it. We do have plenty of access to speeches Sun gave around the Christian world, when he traveled across America in an attempt to garner financial support from Chinese ex-pat communities (Fun fact: in 1911 during the revolution, Sun was actually in Denver, CO). Take for example, in a speech in San Francisco in 1911 just before the revolution:
Our greatest hope is to make the Bible and Christian education, as we have known it, the means of conveying to our countrymen what blessings may be in the way of just laws. - (A Wells, The Influence of Christianity on Sun Yat-sen, 2001)
But is this political pandering, or actual belief? Its a question that has been examined for decades now, with no real conclusive evidence one way or the other. Recent research, however, has argued that Sun's actions can speak for themselves in some ways. Sun never shied away from his acceptance of Christ as a savior. He stated in public that he worshiped Jesus, although he never attended church or religious holidays:
He once asserted that: 'I do not belong to the Christianity of the churches but to the Christianity of Jesus who was a revolutionary. - A Wells
A very interesting statement. Its clear that by his early adult years Sun's view of what Christianity is, is developing along political lines. This is a motif that will appear for the next thirty-some years in the KMT; the exemplification of Jesus the burden sufferer. Guess who else is bearing loads of suffering during this time. Yes, the various leaders of China; in this case Sun, who is attempting to unite and fundamentally change China and in the future Chiang, who is facing certain extermination in the face of Japanese invasion.
Its clear that Sun, like most Chinese Christians during this period, had no interest in ecclesiastical Christianity. But there is some evidence that Sun was a regular reader of the Bible, and he was known to discuss Christian subjects at length. He often made reference to Christian themes: "I am the prodigal son and the lost sheep," Sun once referenced. Some of his actions also hint of Christian values. Sun raised a lot of money for the revolution, but notably died with no personal fortune, and died close to poverty. He was notably kind. After almost thirty years of fighting for a modern China, in 1912 he... renounced the presidency to Yuan Shikai to prevent more violence and again fled to Japan. Does this make him a Christian in private? Again, its hard to tell, but I lay this out for you to make your own judgments. In comparison, Sun was known to use his Christian faith for political gain, as stated above. He even used it to gain local support after being kidnapped in London by the Chinese Legation. But does this make him a false Christian? An opportunist? Naturally, humans are complex, and the more complicated their lives are, the more complex things get.