The first scenario I proposed would get you all of OTL's Christian world, plus all of OTL's Islamic world (including Indonesia), plus any parts of Africa not covered by the aforementioned. I estimate that as being at least four billion people. So that would do the trick (four billion is over 50% of humanity) without involving India and China. In fact, those two plus Indochina would be the only parts of the world that aren't Christian in that scenario-- good for a total of about 3 billion non-Christians by the present day. (Granted, since the scenario covers so long a time, countless things could derail it-- but it's the most feasible way I can think of to meet the OP's challenge.)
Yes, just butterflying Islam and having Christianity take it's place historically seems to meet this challenge.