Sorry its only 1.4 billion my mistake, I should have said "asia". Also though "christianity" isnt a religion if your counting "catholicism, Protestant, Eastern Orthodox,Lutheran,Latter day saints,Jehovah's witnesses, Nominal etc etc" all as the same group. Theres over 50 major "sects" for so called christianity. The only people who claim christianity as a whole is those who wish to push their religion to the largest. (not saying your a "christian trying to do this") But I perused over 40 websites all claiming anyway from 2.5% atheist/agnostic to 20% atheist/agnostic. Also anywhere from 22%-30% "christian". When half the groups included in "christian" don't believe that jesus was their messiah etc (which is the definition of "christianity"), I'm inclined to believe that they (christians) would like to believe they're the biggest but if you go deeper than an actual claim it shows more along the lines of each of those groups (catholicism etc) to be separated into actual different religions and not just "sects" as they dont even come close to believing the same thing, for instance Jehovah's witnesses tend to fall in line with Lutherans, but neither comes close to Roman Catholicism. Well I will admit I was wrong , christianity is certainly not the biggest unless you lump all their denominations together which by definition makes them "non christian" as many don't believe Jesus was their saviour (see above). That would make Muslim the largest as the sects in muslim religions all have the same prophets(but not necessarily the same worldview, but the same religious view nonetheless, unlike those we define as "christian" despite their major differences).

Also you define Chinese as having many other religions ie: buddhism and Taoism unfortunately the adherents of such will tell you one thing. Only a "non buddhist or Taoist" would call it a religion (along with confucianism) when the actual practitioners call it a philosphy something entirely different from religion. Which is also why Taoism and Buddhism are generally not taught in university level Religion courses but instead in Philosphy courses , same with confucianism. Also they account for still less than a quarter of china's 1.4 billion. Also it still has the highest population of any country not one of the largest , it is simply still the largest. And, technically speaking still after all those stats that would still make China "1 billion atheists" despite statistics online from major groups (mostly christian based ironically....)

Plus just saying im "very very" anything doesnt make your point backing it up does though