An interesting essay from Carol Lee Hamrin, coeditor of God and Caesar in China: Policy Implications of Church-State Tensions, via the AEI website:
The number of religious believers in China continues to grow almost exponentially, far outpacing population growth. Meanwhile, vague and unchanging official estimates, which since 1994 have reported “over 100 million faithful” in the country, reflect the government’s tendency to mask the rapid growth evident on the ground. In February 2007, for the first time, the official media reported on an academic challenge to these earlier figures. Scholars in Shanghai had made very rough projections based on a limited survey, suggesting that there were up to 40 million Protestants in China among a total of 300 million religious adherents (not including estimated adherents of informal popular religions or “folk faiths”).
Of the officially tolerated faiths, Christianity has grown at the fastest pace. There were fewer than 1 million Protestants and over 4 million Catholics in 1949–a little over 1 percent of China’s total population of 450 million. By 1965, there were far fewer practicing Christians, of course, as Mao Zedong pursued his policy of escalating persecution. Yet, by 1980, the total was back up to 4 million and growing. As of 2005, Christians were approaching 5 percent of the population, four-fifths of them Protestants, all with virtually no public support or access to China’s mass media–and with the majority not registered with the government. Projections for 2020 show even more growth, with a jump to 10 percent or even more. Unregistered Christians may be the largest autonomous social group in China.
Read also Christianity in China 1900-1950: The History that Shaped the Present by Daniel H. Bays.