That number of 1.8bn does actually seem possible, if one counts as christians anyone who has been baptized whether or not they're actively practising.
Consider that much of South America is intensely religious, and their religion of choice is christianity. The same goes for much of Africa once you get below the Sahara. I would not be at all surprised if these two counted for upwards of a billion between them, if not more. Then add in Europe, North America and any smaller groupings in the rest of the world, and yeah, there could be 1.8bn. Again, assuming the definition is broadened to count anyone who has been baptized.
But of course, being baptized when you were a baby doesn't necessarily mean that you will actually believe in god when you grow up, which leads to a rather large gap between what the various christian denominations think their numbers of followers are, and the number of people who actively follow them.
Anyway, enough of that. Anyone besides me looking forward to seeing how fast the doomsayers can pedal backwards now that, against all odds, the world is still here?

I predict deliciously absurd excuses, and that we'll soon have a new "real" judgement day scheduled to make up for 'calculation errors'.