The split has nothing to do with real world followers, and more to do with Civs picking religions in the game.
At the moment, the preferred religions look like this:
Christianity: 20 Civs
Islam: 6 Civs
Tengriism: 2 Civs
And all the rest are either 1 (Buddhism, Confucianism, Hinduism, Shinto, Taoism, Zoroastrianism) or zero (Judaism or Sikhism).
There's a pretty clear imbalance, so splitting Christianity is a good idea.
Introducing the Christianity split and taking guesses at the current Civs (not including those that will be introduced in BNW) would look something like:
Catholic: 8 Civs
Protestant: 8 Civs
Islam: 6 Civs
Orthodox: 4 Civs
Tengriism: 2 Civs
etc...
And of the known BNW Civs now there will be at least 3 (possibly 4 with Zulu, as the norm for native Civs) Christian countries, probably all Catholic. Assyria will probably be Muslim since all the Middle Eastern civs are, and from out best guesses Morocco and Indonesia would be Muslim, Native Americans and any other European Civ (Italy etc..) would be Christian, wild cards no idea obviously...
So really, looking at the numbers there is more of a reason to split Catholicism again than split Islam. Plus as mentioned splitting Sunni and Shia would make very little difference, as pretty much every country would be Sunni (Persia/Iran is the most Shia country at the moment, but they've got their own religion. Iraq/Babylon might be a push, but I can't honestly see any more).
What it would be good for is if they expand the system so Civs have more than one preferred religion, which they may do now. At the moment America's first choice is Christianity, if (say) England gets that first their second choice is just what's first on the list. Introducing the split in BNW wouldn't change that at all since both America and England will probably be Protestant. What would be best would be if each Civ had an ordered list of preferences, but that would be an ideal scenario.
