To be conservative about it, let’s say 10% of the population is openly or secretly Christian (did your numbers count the Catholics who are also sanctioned, but not part of TSP?).
What will the number look like in 2030?
I haven’t read the whole wiki but this might answer part of that question.
http://en.wikipedia.org/wiki/Christianity_in_China
In 2006 it was stated that there were 4 million members of the Chinese Patriotic Catholic Association and an estimated 12 million members of the underground Roman Catholic Church in China as of 2006.[53] Kiven Choy stated, in a Chinese weekly newspaper in Hong Kong, that the correct number of Protestants in China should be at around 20 million[citation needed], while Time Magazine reported 65 million in 2006.[54]